Adding LICENSE file

[ebezzam]

Perhaps it's worth mentioning that GitHub lets you add a LICENSE file during project creating (so you don't need to manually add it): https://github.com/new

Also it could be nice to specify which license to pick? @fakufaku probably remembers better but I remember someone's feedback about pyroomacoustics was that he would have been happier to see MIT license.

[fakufaku]

GPL is super binding. To the point that some people might not use the software at all. GPL will contaminate any new code that uses it.
MIT is much more permissive. We use it for pyroomacoustics now.