Add Issue Triage Policies to Documentation. [ci skip]

username-removed-386624 requested to merge cs-issue-triage into master

What does this MR do?

It adds Issue Triage Policies to the documentation as discussed in #17693 (closed).

Are there points in the code the reviewer needs to double check?

Make sure none of the policies stick out as unreasonable to you. If they do, I'd appreciate rationale as to why. This is based on what I've learned from closing ~200 issues and reviewing quite a few more.

Other than that, spelling, grammar, etc.

Why was this MR needed?

We have 4000+ issues on the GitLab CE project as of June 30, 2016. That's more than Atom (1721), Rails (458), Ruby (1920), and React (475).

What are the relevant issue numbers?

Closes #17693 (closed).

cc: @axil @virtuacreative @rspeicher @DouweM @JobV @jschatz1 @MrChrisW @dblessing @markpundsack @sytses

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