doc: Document how to check po4a translations for valid POD markup

README.l10n

@@ -76,3 +76,10 @@ Update to 1354t.
   - msgmerge updates your file with the current POT file.
   - msgfmt checks it for validity.
   - msgcat may detect encoding problems.
+
+  In addition to the above, the easiest way to check that the po4a
+  translations have valid POD markup, is to try to build them (please
+  see the Building sections in the README file for further details
+  regarding the setup of the source tree):
+
+  $ make -C man