librhash: Install COPYING file with CMake documentation

When we install using the bundled librhash source, notify users of its
license terms.

Utilities/cmlibrhash/CMakeLists.txt

@@ -34,3 +34,5 @@ include_directories(
   )
 
 add_library(cmlibrhash ${librhash_sources})
+
+install(FILES COPYING README DESTINATION ${CMAKE_DOC_DIR}/cmlibrhash)