Updated contributor branch references - 2.0 no longer maintained

The current documentation refers throughout to making bugfix and
corrections against the 2.0 branch, however this is no longer
maintained. Updated with references to 2.1 to make this more
obvious to new contributors.
[Process] Add documentation about `signal` and `getPid` methods
Update generic_event.rst
Merge pull request #2503 from acoulton/updating-contrib-branch-refs

Updated contributor branch references - 2.0 no longer maintained
Merge branch 'process-signal-pid' of github.com:romainneutron/symfony-docs into romainneutron-process-signal-pid

Conflicts:
	components/process.rst
[#2504] Tweaks to new Process signal and pid docs
[#2504] Adding some better headers to the Process component
Merge branch '2.1' into 2.2

Conflicts:
	components/process.rst
Merge branch '2.2'
Merge branch 'master' into romainneutron-process-signal-pid

Conflicts:
	components/process.rst
[#2504] Backporting information about stopping a process
Merge branch '2.1' into 2.2
Merge branch '2.2'
Merge branch 'master' into romainneutron-process-signal-pid

Conflicts:
	components/process.rst