Add license file

[larsoner]

Most repos have a license file, this one does not. Might as well for consistency!

This is problematic for example when creating a conda-forge package using grayskull I'd need to create this file if it doesn't exist already.

License text copied from https://opensource.org/license/mit and added the year and "surfa developers" as the copyright holder.

Also fixed a bug where the long description content type was not specified.