doc: add CONTRIBUTING.md

CONTRIBUTING.md

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+## Making changes to CitGM
+#### There are a few basic requirements for making changes to CitGM
+
+* Consider creating an issue prior to submitting a PR as this can help speed up
+the process.
+* Include tests for your code wherever possible.
+* Include any documentation or man page changes where necessary.
+* Ensure that `npm test` passes before submitting the PR.
+* Squash each logical change into a single commit.
+* Follow the [commit guidelines](#commit-guidelines) below.
+
+
+#### Commit Guidelines
+
+* The first line should be 50 characters or less and contain a short description
+of the change. All words in the description should be in lowercase with the
+exception of proper nouns, acronyms, and the ones that refer to code, like
+function/variable names. The description should be prefixed with the name of
+the changed subsystem and start with an imperative verb.
+Example: `lookup: add module to lookup.json`. Use `git log` for inspiration if
+you are unsure about what the subsystem should be.
+* Keep the second line blank.
+* Wrap all other lines at 72 columns.
+* Optionally you can include a `PR-URL:` line. (This can be useful when landing
+someone else's commits).
+* If the PR fixes an issue, please include a `Fixes:` or `Closes:` line.
+
+A good commit log can look something like this:
+
+```
+subsystem: explain the commit in one line
+
+Body of commit message is a few lines of text, explaining things
+in more detail, possibly giving some background about the issue
+being fixed, etc.
+
+The body of the commit message can be several paragraphs, and
+please do proper word-wrap and keep columns shorter than about
+72 characters or so. That way, `git log` will show things
+nicely even when it is indented.
+
+PR-URL: https://github.com/nodejs/citgm/pull/314
+Fixes: https://github.com/nodejs/citgm/issues/313
+```
+
+## Submitting a module to CitGM
+[Read the module requirements](README.md#requirements-for-inclusion-in-nodejs-citgm-runs)