Add LICENCE file

[dils2k]

Your docs look amazing! I wanted to know if I can use this repo as a template for my own API documentation websites, but unfortunately I didn't find the LICENCE file. Would be awesome if you added the licence.

[dils2k]

@henryrowett sorry, I didn't know who to mention to get attention to this issue, the last commit was yours, that's why I chose you.