Document submodules in README.md
Document submodules in README.md

From a fresh clone of the rook source, you have to
update the git submodules before building rook.

Fix a bad reference to build/clean, it should
be build/reset

gcmalloc/rook,aramisjohnson/rook,rook/rook,rook/rook,rook/rook,rook/nfs,aramisjohnson/rook,gcmalloc/rook,rook/rook,rook/nfs,aramisjohnson/rook,rook/nfs,rook/rook