CONTRIBUTING.md

Added performance guidelines for new MRs

## What does this MR do?

This MR adds a set of guides that should be followed by merge request authors.

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

Spelling, grammar, etc

## Why was this MR needed?

There is no set of guidelines one should follow when submitting merge requests. This leads to developers at times disregarding performance. This in turn results in performance specialists having to clean up the mess, or production engineers being woken up in the middle of the night because the database is on fire.

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @DouweM @rspeicher @pcarranza @dzaporozhets 

See merge request !5905