Add a basic rule to contributors / PR template. (ray-project#10277)

* Add a basic rule to contributors / PR template.

* Fix.

* Addressed code reivew.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,7 @@
 <!-- Thank you for your contribution! Please review https://github.com/ray-project/ray/blob/master/CONTRIBUTING.rst before opening a pull request. -->
 
+<!-- Please add a reviewer to the assignee section when you create a PR. If you don't have the access to it, we will shortly find a reviewer and assign them to your PR. -->
+
 ## Why are these changes needed?
 
 <!-- Please give a short summary of the change and the problem this solves. -->

CONTRIBUTING.rst

@@ -25,3 +25,22 @@ To contribute a patch:
 .. _`[email protected]`: https://groups.google.com/forum/#!forum/ray-dev
 .. _`GitHub Issues`: https://github.com/ray-project/ray/issues
 .. _`StackOverflow`: https://stackoverflow.com/questions/tagged/ray
+
+PR Review Process
+-----------------
+
+For contributors who are in the ray-project organization:
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- When you first create a PR, add an reviewer to the `assignee` section.
+- Assignees will review your PR and add `@author-action-required` label if further actions are required.
+- Address their comments and remove `@author-action-required` label from the PR.
+- Repeat this process until assignees approve your PR.
+- Once the PR is approved, the author is in charge of ensuring the PR passes the build. Add `test-ok` label if the build succeeds.
+- Committers will merge the PR once the build is passing.
+
+For contributors who are not in the ray-project organization:
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Your PRs will have assignees shortly. Assignees or PRs will be actively engaging with contributors to merge the PR.
+- Please actively ping assignees after you address your comments!