add license, closes #87

As suggested by @jamesarosen: You'll need sign-off from all contributors:

- [x] @hasghari
- [x] @shajith
- [x] @mpapis
- [x] @jish
- [x] @mnzaki
- [x] @jamesbarnett
- [x] @deviantintegral
- [x] @zzet
- [x] @markprzepiora
- [x] @carhartl
- [x] @mikedfunk
- [x] @filipegiusti
- [x] @mikz
- [x]  @jamesarosen