docs and `create` template for new license stanza

This is the first new stanza to be added to the default template in a long time.

At the end of transition, it will become a required stanza.  However, it is perfectly
fine to leave the value as `:unknown`, and we should encourage Cask authors to
do so if there is any difficulty.

Likewise, we have no intention of covering the infinite possible commercial /
closed-source licenses.  If there is no matching license, rather than quibble over
details, the Cask should use `license :commercial` or `license :closed`.