chore(doc): Add add-on install links to README.md
chore(doc): Add add-on install links to README.md

Mardak/activity-stream,mozilla/activity-streams,rlr/activity-streams,AdamHillier/activity-stream,Mardak/activity-stream,JaredKerim-Mozilla/activity-stream,rlr/activity-streams,sarracini/activity-stream,mozilla/activity-stream,Mardak/activity-stream,JaredKerim-Mozilla/activity-stream,AdamHillier/activity-stream,mozilla/activity-stream,AdamHillier/activity-stream,mozilla/activity-streams,JaredKerim-Mozilla/activity-stream,sarracini/activity-stream,JaredKerim-Mozilla/activity-stream,mozilla/activity-stream,sarracini/activity-stream,rlr/activity-streams,mozilla/activity-stream,Mardak/activity-stream,rlr/activity-streams,sarracini/activity-stream,AdamHillier/activity-stream