Update Derek behaviour

- adds new opt-in feature "pr_description_required" see docs for more:
https://github.com/alexellis/derek/blob/master/USER_GUIDE.md

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

.DEREK.yml

@@ -17,3 +17,5 @@ curators:
 
 features:
   - comments
+  - pr_description_required
+