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

When a new PR is opened, Oppiabot leaves a message saying

"Hi @xxx please assign the required reviewer(s) for this PR. Thanks!"

However, for new contributors who don't have "assign" rights, this results in them not being sure what to do.

Describe the solution you'd like

I'd suggest changing the message to:

"Hi @xxx, please assign the required reviewer(s) for this PR. This is necessary for your PR to get reviewed and merged.

Note: If you don't have permissions to modify the "Assignees" section, just leave a comment here saying @{{reviewer_username}} PTAL and I'll help you assign them. Thanks!"