patch 8.0.0830: translating messages is not ideal

Problem:    Translating messages is not ideal.
Solution:   Add a remark about obsolete messages.  Use msgfmt in the check
            script. (Christian Brabandt)
diff --git a/src/po/README.txt b/src/po/README.txt
index a403a2f..8efed96 100644
--- a/src/po/README.txt
+++ b/src/po/README.txt
@@ -84,7 +84,13 @@
     You should include your name and E-mail address instead, for example:
 	msgstr "Berichten übersetzt bei: John Doe <john@doe.org>"
 
-(3) Clean up
+(3) Remove unused messages (optional)
+    Remove messages that have been marked as obsolete. 
+    Such messages start with "#~".
+
+    The cleanup script will also do that (see next step).
+
+(4) Clean up
     This is very important to make sure the translation works on all systems.
     Comment-out all non-translated strings.  There are two types:
     - items marked with "#, fuzzy"