Document LLVM license for binary builds

If we ship llvm as part of / dependency of qdoc in binary packages, we need to provide an appropriate attribution notice in the Qt installer/documentation:

http://releases.llvm.org/4.0.0/LICENSE.TXT

This could arguably be done in the general documentation, where there's precendence for mesa/llvmpipe:

https://doc.qt.io/qt-5/qt-attribution-llvmpipe.html