allow list URL to be omitted

README.md

@@ -61,7 +61,8 @@ class: extra_mailman_archivers.pipermail.Pipermail
 enable: yes
 ```
 
-The default list URL for this archiver is `http://$domain/pipermail/$short_listname`.
+The default list URL for this archiver is `http://$domain/pipermail/$short_listname`
+(Mailman adds a `List-Archive` header to each message with this value).
 The default Mailman 2 directory is `/var/lib/mailman`.
 To change either of these values, add the following to the pipermail
 section in mailman.cfg:
@@ -75,3 +76,8 @@ appropriately, e.g.
 [general]
 base_url: http://$domain/old/pipermail/$short_listname
 ```
+
+To hide the list URL from each message, simply set `base_url` to an empty string:
+```
+base_url:
+```

src/extra_mailman_archivers/pipermail.py

@@ -72,6 +72,9 @@ class Pipermail:
 
     def list_url(self, mlist):
         """See `IArchiver`."""
+        if self.base_url == '':
+            # admin explicitly doesn't want list URL to be advertised
+            return None
         return expand(self.base_url, mlist)
 
     def permalink(self, mlist, msg):