Add LICENSE.TXT file, to be referenced by all files in cling.
Add LICENSE.TXT file, to be referenced by all files in cling.

Dual-license cling under llvm's license and ROOT's LGPL 2.1 - as was originally
the intent.

marsupial/cling,root-mirror/cling,karies/cling,karies/cling,marsupial/cling,karies/cling,karies/cling,root-mirror/cling,perovic/cling,perovic/cling,root-mirror/cling,root-mirror/cling,perovic/cling,marsupial/cling,root-mirror/cling,root-mirror/cling,karies/cling,perovic/cling,marsupial/cling,perovic/cling,marsupial/cling,marsupial/cling,karies/cling