Add a license file.
Add a license file.

The MIT license was previously incorporated by reference in the README,
but it's recommended to include the license text to avoid ambiguity.

https://help.github.com/articles/open-source-licensing

ab/rest-client,jonkgrimes/rest-client,keir-rex/rest-client,voxik/rest-client,skv-headless/rest-client,acrogenesis-lab/rest-client,LYY/rest-client,rayzhng/rest-client,guoyu07/rest-client,sideci-sample/sideci-sample-rest-client,antonklava/rest-client,crccw/rest-client,taoza/rest-client,rest-client/rest-client,JIFFinc/rest-client,korczis/rest-client,cchandler/rest-client,landylee1989/rest-client,maxprig/rest-client