Add macro add_action_files to CMakeLists.txt.in and a hint on how to enable message generation

To make it easier for people to enable generation of messages or actions to a package without having to consult the wiki again, if the only thing one cannot remember is where to depend on which package to just make it work.
Merge pull request #67 from felix-kolbe/patch-1

Add add_action_files macro to CMakeLists.txt.in and a depends-hint