Add LICENSE.txt (MIT)

Added a LICENSE.txt (MIT). That's the "default" per https://wiki.jenkins-ci.org/display/JENKINS/Before+starting+a+new+plugin but it is better (legally) to have an explicit license than relying on something written on some web page on the internet. License are for lawyers and lawyers prefer the explicit way, sadly.

The pom.xml is already specifying the MIT license, so this change is a no-op. 

Please kindly accepts this pull request since it will make it easier for us (and other companies that employ a bunch of lawyers) to use your fine software :)