docs: Add contribution guidelines. (#2045)

Following b/323273911 and the guidance in the ticket.

---------

Co-authored-by: Lawrence Qiu <lawrenceqiu@google.com>

CONTRIBUTING.md

@@ -1,7 +1,11 @@
 # How to Contribute
 
-We'd love to accept your patches and contributions to this project. There are
-just a few small guidelines you need to follow.
+We'd love to accept your patches and contributions to this project. There are just a few small guidelines you need to follow before opening an issue or a PR:
+1. Ensure the issue was not already reported.
+2. Open a new issue if you are unable to find an existing issue addressing your problem. Make sure to include a title and clear description, as much relevant information as possible, and a code sample or an executable test case demonstrating the expected behavior that is not occurring.
+3. Discuss the priority and potential solutions with the maintainers in the issue. The maintainers would review the issue and add a label "Accepting Contributions" once the issue is ready for accepting contributions.
+4. Open a PR only if the issue is labeled with "Accepting Contributions", ensure the PR description clearly describes the problem and solution. Note that an open PR without an issue labeled with "Accepting Contributions" will not be accepted.
+
 
 ## Contributor License Agreement