COPYING: add CC-BY-SA-3.0 and GPLv3 license texts

In preparation of making the switch.
COPYING: The wiki is now dual licensed under CC-BY-SA-3.0 and GPL v3+

All the content in the wiki now falls into one of these categories, in
decreasing order of quantity:

* The authors have agreed that all of their contributions to the notmuch
  wiki can be dual licensed under CC-BY-SA-3.0 and GPL v3+.

* The content is copied from the notmuch main repository and is licensed
  under GPL v3+. Effectively this is the man pages.

* The authors have not been reached but their contribution have been a
  very minor or trivial change or cleanup of content falling into above
  categories.

* The authors have not been reached but the content has been deleted.