doc: update CONTRIBUTING.md

add example and link to help new contributor to know how to write commit
message and sign it off.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

CONTRIBUTING.md

@@ -108,6 +108,7 @@ Actually in PouchContainer, we take two rules serious when committing:
 
 * [Commit Message](#commit-message)
 * [Commit Content](#commit-content)
+* [Sign your work](#sign-your-work)
 
 #### Commit Message
 
@@ -126,6 +127,8 @@ On the other side, we discourage contributors from committing message like the f
 * ~~update~~
 * ~~add doc~~
 
+If you get lost, please see [How to Write a Git Commit Message](http://chris.beams.io/posts/git-commit/) for a start.
+
 #### Commit Content
 
 Commit content represents all content changes included in one commit. We had better include things in one single commit which could support reviewer's complete review without any other commits' help. In another word, contents in one single commit can pass the CI to avoid code mess. In brief, there are two minor rules for us to keep in mind:
@@ -137,6 +140,59 @@ In addition, in the code change part, we suggest that all contributors should re
 
 No matter commit message, or commit content, we do take more emphasis on code review.
 
+#### Sign your work
+
+The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch.
+The rules are pretty simple: if you can certify the below (from [developercertificate.org](http://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+ have the right to submit it under the open source license
+ indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+ of my knowledge, is covered under an appropriate open source
+ license and I have the right under that license to submit that
+ work with modifications, whether created in whole or in part
+ by me, under the same open source license (unless I am
+ permitted to submit under a different license), as indicated
+ in the file; or
+
+(c) The contribution was provided directly to me by some other
+ person who certified (a), (b) or (c) and I have not modified
+ it.
+
+(d) I understand and agree that this project and the contribution
+ are public and that a record of the contribution (including all
+ personal information I submit with it, including my sign-off) is
+ maintained indefinitely and may be redistributed consistent with
+ this project or the open source license(s) involved.
+```
+
+Then you just add a line to every git commit message:
+
+```
+Signed-off-by: Joe Smith <joe.smith@email.com>
+```
+
+Use your real name (sorry, no pseudonyms or anonymous contributions.)
+
+If you set your `user.name` and `user.email` git configs, you can sign your commit automatically with `git commit -s`.
+
 ### PR Description
 
 PR is the only way to make change to PouchContainer project files. To help reviewers better get your purpose, PR description could not be too detailed. We encourage contributors to follow the [PR template](./.github/PULL_REQUEST_TEMPLATE.md) to finish the pull request.