Add infrastructure requirement

wiki/src/doc/about/openpgp_keys/signing_key_revocation.mdwn

@@ -77,6 +77,8 @@ Infrastructure
   - Everybody who owns a share is subscribed to a mailing list.
   - This mailing list is hosted on a trusted server different from boum.org to
     be more resilient than our usual communication channels.
+  - Someone hosting the mailing list is part of group B, C, or D so they
+    can ensure that the list keeps working even if it never used.
 
 Changing the members of the groups B, C, or D
 =============================================