CC-BY-3.0-short.md: add CC-BY 3.0 unported summary

CC-BY-3.0-short.md: add CC-BY 3.0 unported summary

The summary is owned by Creative Commons and is distributed under the
CC BY 3.0 unported.

From http://creativecommons.org/terms:
> 8. Terms Relating to Content on the Websites and Services.
>    b. Licenses Associated With Content on the Websites and Services.
>       i. CC Content: All Content (other than computer software) owned
>          by Creative Commons and made available by CC on the Websites
>          or through the Services is licensed under the Creative
>          Commons Attribution 3.0 Unported license, unless marked
>          otherwise.

I've converted the HTML version from
http://creativecommons.org/licenses/by/3.0/ to markdown, but haven't
made any other changes.

Signed-off-by: W. Trevor King <wking@tremily.us>