add license

[jseldess]

Add creative commons license.

Note from @bdarnell:

• Attribution (CC BY): the most permissive license, and the closest to the apache software license
• Attribution-ShareAlike (CC BY-SA): GPL-like, prevents closed derivatives of the docs
• Attribution-NonCommercial (CC BY-NC): prevents others from e.g. publishing our docs in book form
• Attribution-NoDerivs (CC BY-ND): prevents anything but a verbatim copy of the docs
• Various combinations of ShareAlike, NonCommercial, NoDerivs

For comparison, mongodb's docs are under ShareAlike-NonCommercial; I checked cassandra/datastax and hbase but it was not immediately obvious what license their docs used. Tornado uses Attribution-only.

Note from @petermattis: The CC BY license sounds good to me. If Apache-style restrictions (or lack thereof) are good enough for our code they should be good enough for our docs.