add LICENSE file
This MR adds an MIT license file with Bat as the copyright holder.
The old module-image for the shared runners will be discontinued on October 31. All users still referencing gitlab-registry.mpcdf.mpg.de/mpcdf/module-image in their CI pipelines need to switch to the new CI images now, see instructions here.
This MR adds an MIT license file with Bat as the copyright holder.