[community goal] Update contributor documentation

This patch updates/adds the contributor documentation to follow
the guidelines of the Ussuri cycle community goal[1].

[1] https://governance.openstack.org/tc/goals/selected/ussuri/project-ptl-and-contrib-docs.html

Story: #2007236
Task: #38555
Change-Id: Ib1578376a5231560cad858274348d8f7dd2b41f2

Author: gmaanos

CONTRIBUTING.rst

@@ -1,2 +1,19 @@
-Please refer to Rally contribution guide: https://docs.openstack.org/developer/rally/contribute.html
+The source repository for this project can be found at:
 
+   https://opendev.org/openstack/rally-openstack
+
+Pull requests submitted through GitHub are not monitored.
+
+To start contributing to OpenStack, follow the steps in the contribution guide
+to set up and use Gerrit:
+
+   https://docs.openstack.org/contributors/code-and-documentation/quick-start.html
+
+Bugs should be filed on Launchpad:
+
+   https://bugs.launchpad.net/rally-openstack
+
+For more specific information about contributing to this repository, see the
+rally contributor guide:
+
+   https://rally.readthedocs.io/en/latest/contribute.html