Add a license

[patcon]

Re-ticketed from https://github.com/tlrobinson/metabase-http-driver/issues/1#issuecomment-603185431

Let me know your preferred license @tlrobinson, and I can submit a PR.

Also, will prob be of interest to @thucnc, as i believe they're building on your code.

Thanks for everything, all!

cc: @pgroenin