Replace the stub README with a proper one based on Clang's.
drwxr-xr-x - clang-tools-extra
drwxr-xr-x - clang
drwxr-xr-x - compiler-rt
drwxr-xr-x - debuginfo-tests
drwxr-xr-x - libclc
drwxr-xr-x - libcxx
drwxr-xr-x - libcxxabi
drwxr-xr-x - lld
drwxr-xr-x - lldb
drwxr-xr-x - llvm
drwxr-xr-x - polly