Add LICENSE.md file

@dancornell,

I have added a LICENSE.md file i.e. https://help.github.com/articles/open-source-licensing#how-can-i-go-back-through-my-public-repositories-and-give-them-licenses

It's text was copied and pasted from http://creativecommons.org/licenses/by-sa/3.0/legalcode as specified within https://github.com/OWASP/SSVL/blob/master/LICENSE