Add a specific open source license and LICENSE file

[philipmathieuKM]

The publication references this repository as "open-source code" but this repository does not contain a specific license or license file. Adding a license file would, in particular, help companies considering using this code in a commercial context ensure that they are abiding by any terms that the author places on open source use. I would suggest the MIT License for its simplicity (both in application and interpretation), but you may have other preferences.