Mention not accepting new addons/modes in CONTRIBUTING.md
Mention not accepting new addons/modes in CONTRIBUTING.md

pabloferz/CodeMirror,SidPresse/CodeMirror,jayaprabhakar/CodeMirror,mtaran-google/CodeMirror,namin/CodeMirror,namin/CodeMirror,grzegorzmazur/CodeMirror,codio/CodeMirror,jkaplon/CodeMirror,wingify/CodeMirror,notepadqq/CodeMirror,dperetti/CodeMirror,nightwing/CodeMirror,SidPresse/CodeMirror,hackmdio/CodeMirror,pabloferz/CodeMirror,MarcelGerber/CodeMirror,ficristo/CodeMirror,hackmdio/CodeMirror,MarcelGerber/CodeMirror,hackmdio/CodeMirror,dperetti/CodeMirror,jkaplon/CodeMirror,kerabromsmu/CodeMirror,thomasjm/CodeMirror,namin/CodeMirror,mtaran-google/CodeMirror,jayaprabhakar/CodeMirror,nightwing/CodeMirror,mtaran-google/CodeMirror,kerabromsmu/CodeMirror,ficristo/CodeMirror,wingify/CodeMirror,MarcelGerber/CodeMirror,notepadqq/CodeMirror,wingify/CodeMirror,ficristo/CodeMirror,grzegorzmazur/CodeMirror,jkaplon/CodeMirror,codio/CodeMirror,dperetti/CodeMirror,nightwing/CodeMirror,SidPresse/CodeMirror,thomasjm/CodeMirror,thomasjm/CodeMirror,codio/CodeMirror,kerabromsmu/CodeMirror,pabloferz/CodeMirror,notepadqq/CodeMirror,jayaprabhakar/CodeMirror,grzegorzmazur/CodeMirror