Add contribution doc.

Fix #CE-3.

Change-Id: I9cf8626fac196c6f21beba792bc441bb454d926a
Signed-off-by: Haitao Yue <hightall@me.com>

README.md

@@ -37,6 +37,7 @@ You can also find more [scenarios](docs/scenario.md).
 * [Architecture Design](docs/arch.md)
 * [Database Model](docs/db.md)
 * [API](api/restserver_v2.md)
+* [How to contribute](docs/CONTRIBUTING.md)
 
 ## TODO
 * restserver: update api definitions yml files.

docs/CONTRIBUTING.md

@@ -0,0 +1,83 @@
+# Contribution
+Any kind of contribution is encouraged, e.g., bug report, question answer, and submit pull-request.
+
+Before taking actions, we highly recommend reading the [docs](../README.md).
+
+
+## Bug and Questions
+
+We now have two channels for bug and questions:
+
+* [Jira](https://jira.hyperledger.org/projects/CE/issues): report bug issues, create to-do tasks.
+* [Slack](https://hyperledgerproject.slack.com/archives/cello): technical discussions and questions.
+
+Jira tasks with `To Do` status are available for picking. If you want to handle one, assign it to yourself, and update the status to `In Progress`. Remember to mark it to `Done` when the patch is merged.
+
+## Code Commit
+
+*Before committing code, please go to [Jira](https://jira.hyperledger.org/secure/RapidBoard.jspa?rapidView=85) to check existing tasks.*
+
+The project employs [Gerrit](https://gerrit.hyperledger.org) as the code commit/review system.
+
+* Clone the project with your Linux Foundation ID (`LFID`), we suggest clone it into the `$GOPATH/src/github.com/hyperledger` directory so that it will build.
+
+```sh
+$ git clone ssh://LFID@gerrit.hyperledger.org:29418/cello && scp -p -P 29418 LFID@gerrit.hyperledger.org:hooks/commit-msg cello/.git/hooks/
+```
+
+(Optionally) Config your git name and email if not setup previously.
+
+```sh
+$ git config user.name "your name"
+$ git config user.email "your email"
+```
+
+(Optionally) Setup git-review by inputting your LFID. Notice this is only necessary once.
+```sh
+$ git review -s
+```
+
+* Create a descriptively-named branch off of your cloned repository
+
+```sh
+$ cd cello
+$ git checkout -b issue-NNNN
+```
+
+* After change, run `make check` to make sure the checking is passed. Then Commit your code with `-s` to sign-off, and `-a` to automatically add changes.
+
+```sh
+$ make check
+  ...
+  py27: commands succeeded
+  py30: commands succeeded
+  py35: commands succeeded
+  flake8: commands succeeded
+  congratulations :)
+
+$ git commit -s -a
+```
+
+Example commit msg may look like:
+
+```sh
+A short description of your change with no period at the end
+
+You can add more details here in several paragraphs, but please keep each line
+width less than 80 characters. A bug fix should include the issue number.
+
+Fix Issue #7050.
+
+Change-Id: IF7b6ac513b2eca5f2bab9728ebd8b7e504d3cebe1
+Signed-off-by: Your Name <committer@email.address>
+```
+
+* Submit your commit using `git review`.
+
+```sh
+$ git review
+```
+
+After the review is uploaded successfully, you can open [Gerrit Dashboard](https://gerrit.hyperledger.org/r/#/dashboard/self) to invite reviewers for checking. The patch will be merged into the `master` branch if passing the reviewer checking.
+
+* If you need to refine the patch further, you can commit the new code with `git commit -a --amend`, and then repeat the `git review` command.