Signed-off-by: Rémy Coutable <remy@rymai.me>

Add link to article on how to write reliable, asynchronous tests with Capybara

This article has been incredibly useful for all developers. Adding a link to it so more people can refer to it.

See merge request !6312

[ci skip]

Signed-off-by: Rémy Coutable <remy@rymai.me>

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

Change to use a better title style

Add backticks to code segments

Spelling and Grammar

Adjust bug template to adhere to standard bug report format

* https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines
* Guidelines use the following format

- Summary
- Steps to Reproduce
- Expected Behaviour
- Actual Behaviour

These guidelines cover the performance requirement for newly submitted
merge requests. These guidelines are put in to place to prevent merge
requests from negatively impacting GitLab performance as much as
possible.