Add License

[RossBarnie]

Without a license it's not clear what rights the authors, contributors or users have to copy/distribute or use the software, see https://choosealicense.com/no-permission/.

If I've simply missed it please let me know but I couldn't see it in any of the conventional places such as in a LICENSE or CONTRIBUTING file.

[kyleam]

I very much agree that there should be a top-level license file. A good number of the files have headers with an Apache license statement.