.pullapprove.yml: Add an always_pending setting

Docs in http://docs.pullapprove.com/groups/always_pending/

The ^ prefix (anchoring the match to the front of the title) is
Aleksa's suggestion [1].

[1]: opencontainers#29 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

.pullapprove.yml

@@ -4,6 +4,10 @@ requirements:
   signed_off_by:
     required: true
 
+always_pending:
+  title_regex: ^WIP
+  explanation: 'Work in progress...'
+
 group_defaults:
   required: 2
   approve_by_comment: