Make the labeling process more apparent

I wanted to label an issue that I created and searched in CONTRIBUTING.md to find some info. It's not clear that we list the labels in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/PROCESS.md#workflow-labels and then there are not all of them. Instead we explain them in the handbook in https://about.gitlab.com/handbook/engineering/workflow/#labelling-issues.

I believe we can make it more clear in CONTRIBUTING.md and link to the handbook even if it is intended for "internal" use. People would still find it interesting and helpful what criteria we use to label things.

cc @connorshea @markglenfletcher @mattl

@axil Good idea, we should try to have a single source of truth for the labelling process/workflow!

I think there was an MR before that @connorshea tried to get in contributing.md, but we decided some things were only geared towards team members and not the community in general. So we documented things in the handbook. I think at least the labels should be in one place :)

I propose that all labels should be in contributing.md, even if some are for team members only. Or as at least clearly crosslink.

@axil Fastlane does a pretty good job with this: https://github.com/fastlane/fastlane/blob/master/GitHubLabels.md

@connorshea I'm all in doing something similar with fastlane. We already have descriptions in labels, but seeing all of them at once in a table surely helps.

Linking these together! https://gitlab.com/gitlab-com/marketing/issues/677

Someone linked this in a Slack group I'm in and I thought this was a pretty clean approach: https://drive.google.com/file/d/0BwZ8RiZCRZzrWUpaZUNEYmlTX3c/view

I particularly like that they're color coded so people know what colors to look out for too. Looks like we do a little bit of that but it would be nice to have consistency.

mentioned in issue #29030 (closed)

mentioned in commit 1f2bedf3

closed via merge request !9850 (merged)

mentioned in commit f67019ad

mentioned in merge request !14785