Add contributing guidelines
Various typo fixes suggested by @renelvon
Improve CONTRIBUTING.md

* Add sections about code and PR documentation.
* Restructure finding-an-issue section.
* All kind of style fixing
Merge pull request #1 from Renelvon/contributing

Restructure and expand CONTRIBUTING
add a link to the hub repo
Add links to Workflow and Reviewing Pull Requests sections
Suggest multiple pull requests for big features
Merge pull request #617 from dionyziz/contributing

Contributing guidelines
Revert "Revert PR #621"

This reverts commit d32db58bb3f693de33776229c76185d270d32a6e.
Allow non relative imports for now (W0403).
Revert "W0403: Honour relative imports."

Conflicts:
	features/environment.py

Conflicts due to util.setup_db already dropped as empty import.
Do not cancel entire checksuite if pylint is absent.