* Adopted both versions 3.0 and 4.0 (i.e. 3.0 or any later) of the CC
BY-SA license.
* Added copies of each to the repo as well as copies converted from HTML
to Org-Mode.
* Converted copies might require some editing, but they'll do for
now (at least full copies are included at all, most projects just link
to them).
* Updated README to reflect this change.
* Included external links in the README.
