Add new Makefile step "merge" and document it.

Makefile

@@ -37,6 +37,10 @@ help:
 pot: 
 	xgettext --keyword=__ --keyword=___ --from-code=UTF-8 --output=locale/converse.pot converse.js --package-name=Converse.js --copyright-holder="Jan-Carel Brand" --package-version=0.4 -c --language="python";
 
+merge:
+	find ./locale -maxdepth 1 -mindepth 1 -type d -exec msgmerge {}/LC_MESSAGES/converse.po ./locale/converse.pot -U \;
+
+
 release: 
 	r.js -o build.js
 

docs/source/index.rst

@@ -604,7 +604,13 @@ You can then create or update the PO file for a specific language by doing the f
 
     msgmerge ./locale/de/LC_MESSAGES/converse.po ./locale/converse.pot -U
 
-This PO file is then what gets translated.
+To do this for ALL languages, run:
+
+::
+
+    make merge
+
+The resulting PO file is then what gets translated.
 
 If you've created a new PO file, please make sure to add the following
 attributes at the top of the file (under *Content-Transfer-Encoding*). They are