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