Add a note about erlang-os-mon and similar

Author: josevalim

guides/os_mon.md

@@ -15,6 +15,8 @@ The OS Data comes from the `os_mon` application, which ships as part of your Erl
   end
 ```
 
+> Some operating systems break Erlang into multiple packages. In this case, you may need to install a package such as `erlang-os-mon` or similar.
+
 ## Configuring os_mon
 
 See [the Erlang docs](http://erlang.org/doc/man/os_mon_app.html) for more information and `os_mon` configuration.