Add LICENSE File

Historically, it's been a pain to use MPL on projects due to a per-file header requirement. However, I noticed this clause in the license (emphasis mine):

> If it is not possible or **desirable** to put the notice in a 
> particular file, then You may include the notice in a location 
> (such as a LICENSE file in a relevant directory) where a recipient 
> would be likely to look for such a notice.

As I find this requirement to be undesirable, thus we are placing the notice in this file.