Add brief information about labels/colors used by the bots

[willingc]

From the Zulip chat:

Ah, I didn't notice that awaiting review and awaiting merge are different states -- they have the same color. :-( It would be nice if all this was written up in the devguide, or at least the devguide had a pointer to some comment in bedevere's source where it's explained.

Write up info about labels and colors for the amazing @Mariatta's bots.

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#32 (Add the mention bot?), python/miss-islington#43 (Add info about how to customize and use this bot for other repositories?), python/core-workflow#239 (Bot for Github labels), #396 (Add a few missing in-use directives), and #296 (Add a section to explain the different GitHub PR labels).

[mariatta-bot]

Mariatta was mentioned, but she's out of open source for the rest of September 2018. Be aware she might not get to this until October 2018.
(I'm a bot)

[Mariatta]

The auto labeling by bedevere was actually @brettcannon's work :) so I don't want to take any credit here.

I think it is duplicate of #296 though, or similar.

[brettcannon]

I agree that this is a duplicate of #296 .