README.rst: Add license badge
This will add a license badged from the analisys done at https://app.fossa.io/projects/git%2Bgitlab.com%2FBuildStream%2Fbuildstream/refs/branch/master/
Edited by Javier Jardón
This will add a license badged from the analisys done at https://app.fossa.io/projects/git%2Bgitlab.com%2FBuildStream%2Fbuildstream/refs/branch/master/