Coala: Newcomers guide: Add introduction to Gitter

[NiklasMM]

A Gitter section should be added to the newcomers guide, probably in "Step 1: Meet the community" or as a separate page linked from there.

It should include:

• Short intro (What is Gitter?)
• Explain which channels are there. (Including non-repo related ones like offtopic, conferences....)
• Explain Gitter Etiquette, i.e. don't @-mention or private message people unless they are the only one who could help you. Don't continuously @-mention in an ongoing conversation.

[adtac]

Maybe also add "do not assign issues through cobot unless you entirely know what the issue is about and how to solve it" (I know it's already in the newcomer invite, but nobody follows it anyway)

[NiklasMM]

Yes, maybe we could add a short intro to cobot there also!

[khushmeeet]

I would like to be assigned to this issue.

[supergr35]

I would really like a short intro to cobot, especially highlighting: issue assignment, marking things wip/pending, and creating issues.