Update CONTRIBUTING.md

merged

mentioned in commit 54fb5b6e

added Documentation label

@rmatev I did not touch the last paragraph concerning the use of WIP, but it should probably be updated. Is there maybe a reference that can be made to the RTA code review process?

One can consider linking to the shifter

https://twiki.cern.ch/twiki/bin/viewauth/LHCb/RealTimeAnalysisShifterChecklist

and maintainer

https://twiki.cern.ch/twiki/bin/view/LHCb/RealTimeAnalysisMaintainerManual

manuals? Might be too much information for most contributors, but preferable imo to duplicating information because then we risk giving contradictory advice.