Add section about contributing to README

Closes #23830 (closed)

@axil I added the section before "Install a development environment", because installing a development environment only makes sense when you want to contribute, so contributing should go logically first.

Restored source branch link-to-contributing-from-readme

Milestone changed to %8.14

Perfect! Thanks :)

Enabled an automatic merge when the build for 48649715 succeeds

Canceled the automatic merge

Status changed to merged

Mentioned in commit 09cccd93