Add a license file?

[IanLee1521]

@lee218llnl is this something that you can do?

[lee218llnl]

I will defer this to @simoatze

[simoatze]

Do we need one? I did this just to be able to build "FileCheck" and "llvm-lit" since they don't get installed with LLVM/Clang, so one can still use the LLVM/Clang binary and only compile those two tools.
If a license file is necessary what should I do? Add the same license as Archer?