Is your feature request related to a problem? Please describe.

The wiki page https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#clarification-regarding-assignees-and-reviewers-section explains what to do when sending a PR for review. However, new contributors don't have permissions to update the assignee fields in GitHub, so the instructions aren't followable as-is.

Describe the solution you'd like

Explain, in that section, how to add assignees in such cases (e.g. explaining how to leave a comment like "@Reviewer PTAL" so that Oppiabot assigns the reviewer for them).