Document and example of pre_uninstall/0 (processone/ejabberd#3548)

README.md

@@ -64,3 +64,7 @@ The following organization has been set up for the development:
 
 - Module developers should note in the `README.txt` file whether the
   module has requirements or known incompatibilities with other modules.
+
+- If your module project contains several erlang modules, you should export a
+  function pre_uninstall/0 in the main one listing the other ones.
+  See mod_statsdx as an example.

mod_statsdx/src/mod_statsdx.erl

@@ -15,6 +15,7 @@
 
 -export([start/2, stop/1, depends/2, mod_opt_type/1, mod_options/1]).
 -export([loop/1, get_statistic/2,
+	 pre_uninstall/0,
 	 received_response/3,
 	 %% Commands
 	 getstatsdx/1, getstatsdx/2,
@@ -79,6 +80,10 @@ stop(Host) ->
 	_ -> ?PROCNAME ! {stop, Host}
     end.
 
+pre_uninstall() ->
+    [{code:purge(M), code:delete(M)}
+     || M <- [mod_stats2file]].
+
 depends(_Host, _Opts) ->
     [].