[llvm-lipo] Add docs for llvm-lipo

Add docs (llvm-lipo.rst) for llvm-lipo.

Test plan:
make -j8 sphinx 
check that ./docs/html/CommandGuide/llvm-lipo.html is built correctly and looks okay.

Differential revision: https://reviews.llvm.org/D62706

llvm-svn: 362848