R1:d5dedda84317

Document the IRC bot

Summary: Write a little documentation about how to get the IRC bot running since
there's a reasonable process with some examples but no documentation.

Test Plan: Generated, read the documentation.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: iAladdin, aran, jungejason

Maniphest Tasks: T686

Differential Revision: 1244