docs: commit message labels
docs: commit message labels

* NEW Documents commit message labels used by developers (such as NEW,
  SECURITY, FIX, etc.) used in automatic generation of structured
  release notes.  (closes #2856)

Reviewed-by: Samuele Kaplun <2e218f779e12714c314fad8ebf6b3041daa21337@cern.ch>
Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio