Added CC-BY-SA 4.0 license declaration.

I have added the [CC-BY-SA 4.0 license](http://creativecommons.org/licenses/by-sa/4.0/).
This allows for:

+ *Share*: copy and redistribute the material in any medium or format
+ *Adapt*: remix, transform, and build upon the material

for any purpose, even commercially.

This should address Issue #1

The licensor cannot revoke these freedoms as long as you follow the license terms.