Adding a section to outline the exact process the documentation team takes during the release lifecycle
[#2720] Tweaks thanks to @WouterJ
fix branch version used to base pull requests on
Merge pull request #2720 from symfony/documentation-release-process

The Documentation Release Process
Merge pull request #2758 from xabbuh/issue-2757

fix branch version used to base pull requests on
Merge branch '2.2' into 2.3

Conflicts:
	contributing/documentation/overview.rst
[#2690][#2720][#2758] Updating README for new PR merge version - taken from @WouterJ's PR
Merge branch '2.2' into 2.3