doc: clarified & split up contribution docs

COLLABORATOR_GUIDE.md

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+# io.js Collaborator Guide
+
+This document contains information for Collaborators of the io.js
+project regarding maintaining the code, documentation and issues.
+
+Collaborators should be familiar with the guidelines for new
+contributors in [CONTRIBUTING.md](./CONTRIBUTING.md) and also
+understand the project governance model as outlined in
+[GOVERNANCE.md](./GOVERNANCE.md).
+
+## Issues and Pull Requests
+
+Courtesy should always be shown to individuals submitting issues and
+pull requests to the io.js project.
+
+Collaborators should feel free to take full responsibility for
+managing issues and pull requests they feel qualified to handle, as
+long as this is done while being mindful of these guidelines, the
+opinions of other Collaborators and guidance of the TC.
+
+Collaborators may **close** any issue or pull request they believe is
+not relevant for the future of the io.js project. Where this is
+unclear, the issue should be left open for several days to allow for
+additional discussion. Where this does not yield input from io.js
+Collaborators or additional evidence that the issue has relevance, the
+issue may be closed. Remember that issues can always be re-opened if
+necessary.
+
+## Accepting Modifications
+
+All modifications to the the io.js code and documentation should be
+performed via GitHub pull requests, including modifications by
+Collaborators and TC members.
+
+All pull requests must be reviewed and accepted by a Collaborator with
+sufficient expertise who is able to take full responsibility for the
+change. In the case of pull requests proposed by an existing
+Collaborator, an additional Collaborator is required for sign-off.
+
+In some cases, it may be necessary to summon a qualified Collaborator
+to a pull request for review by @-mention.
+
+If you are unsure about the modification and are not prepared to take
+full responsibility for the change, defer to another Collaborator.
+
+Before landing pull requests, sufficient time should be left for input
+from other Collaborators. Leave at least 48 hours during the week and
+72 hours over weekends to account for international time differences
+and work schedules. Trivial changes (e.g. those which fix minor bugs
+or improve performance without affecting API or causing other
+wide-reaching impact) may be landed after a shorter delay.
+
+Where there is no disagreement amongst Collaborators, a pull request
+may be landed given appropriate review. Where there is discussion
+amongst Collaborators, consensus should be sought if possible. The
+lack of consensus may indicate the need to elevate discussion to the
+TC for resolution (see below).
+
+All bugfixes require a test case which demonstrates the defect. The
+test should *fail* before the change, and *pass* after the change.
+
+### Involving the TC
+
+Collaborators may opt to elevate pull requests or issues to the TC for
+discussion by assigning the ***tc-agenda*** tag. This should be done
+where a pull request:
+
+- has a significant impact on the codebase,
+- is inherently controversial; or
+- has failed to reach consensus amongst the Collaborators who are
+  actively participating in the discussion.
+
+The TC should serve as the final arbiter where required.
+
+## Landing Pull Requests
+
+Always modify the original commit message to include additional meta
+information regarding the change process:
+
+- A `Reviewed-By: Name <email>` line for yourself and any
+  other Collaborators who have reviewed the change.
+- A `PR-URL:` line that references the full GitHub URL of the original
+  pull request being merged so it's easy to trace a commit back to the
+  conversation that lead up to that change.
+- A `Fixes: X` line, where _X_ is either includes the full GitHub URL
+  for an issue, and/or the hash and commit message if the commit fixes
+  a bug in a previous commit.
+
+Additionally:
+
+- Double check PR's to make sure the person's _full name_ and email
+  address are correct before merging.
+- Except when updating dependencies, all commits should be self
+  contained.  Meaning, every commit should pass all tests. This makes
+  it much easier when bisecting to find a breaking change.
+
+### Technical HOWTO
+
+_Optional:_ ensure that you are not in a borked `am`/`rebase` state
+
+```text
+$ git am --abort
+$ git rebase --abort
+```
+
+Checkout proper target branch
+
+```text
+$ git checkout v1.x
+```
+
+Update the tree
+
+```text
+$ git fetch origin
+$ git merge --ff-only origin/v1.x
+```
+
+Apply external patches
+
+```text
+$ curl https://github.com/iojs/io.js/pull/xxx.patch | git am --whitespace=fix
+```
+
+Check and re-review the changes
+
+```text
+$ git diff origin/v1.x
+```
+
+Check number of commits and commit messages
+
+```text
+$ git log origin/v1.x...v1.x
+```
+
+If there are multiple commits that relate to the same feature or
+one with a feature and separate with a test for that feature -
+you'll need to squash them (or strictly speaking `fixup`).
+
+```text
+$ git rebase -i origin/v1.x
+```
+
+This will open a screen like this (in the default shell editor):
+
+```text
+pick 6928fc1 crypto: add feature A
+pick 8120c4c add test for feature A
+pick 51759dc feature B
+pick 7d6f433 test for feature B
+
+# Rebase f9456a2..7d6f433 onto f9456a2
+#
+# Commands:
+#  p, pick = use commit
+#  r, reword = use commit, but edit the commit message
+#  e, edit = use commit, but stop for amending
+#  s, squash = use commit, but meld into previous commit
+#  f, fixup = like "squash", but discard this commit's log message
+#  x, exec = run command (the rest of the line) using shell
+#
+# These lines can be re-ordered; they are executed from top to bottom.
+#
+# If you remove a line here THAT COMMIT WILL BE LOST.
+#
+# However, if you remove everything, the rebase will be aborted.
+#
+# Note that empty commits are commented out
+```
+
+Replace a couple of `pick`s with `fixup` to squash them into a
+previous commit:
+
+```text
+pick 6928fc1 crypto: add feature A
+fixup 8120c4c add test for feature A
+pick 51759dc feature B
+fixup 7d6f433 test for feature B
+```
+
+Replace `pick` with `reword` to change the commit message:
+
+```text
+reword 6928fc1 crypto: add feature A
+fixup 8120c4c add test for feature A
+reword 51759dc feature B
+fixup 7d6f433 test for feature B
+```
+
+Save the file and close the editor, you'll be asked to enter new
+commit message for that commit, and everything else should go
+smoothly. Note that this is a good moment to fix incorrect commit
+logs, ensure that they are properly formatted, and add `Reviewed-By`
+line.
+
+Time to push it:
+
+```text
+$ git push origin v1.x
+```