Minor additions and edits to the v2.16 Release Notes

Add: notes on New configuration option to ignore self approval on label
Add: notes on CodeMirror plugin for PolyGerrit
Add: additional notes on Migration to NoteDb
Various formatting and style changes

Change-Id: Ib46cbf058f5ae3f1a5f05dd0ef5578c18be91e5e
1 file changed