Merge pull request #2795 from edx/derf/pull_request_template

add pull request template

.github/PULL_REQUEST_TEMPLATE.md

+Configuration Pull Request
+---
+
+Make sure that the following steps are done before merging
+
+  - [ ] @devops team member has commented with :+1:
+  - [ ] are you adding any new default values that need to be overriden when this goes live?  
+    - [ ] Open a ticket (DEVOPS) to make sure that they have been added to secure vars.
+    - [ ] Add an entry to the CHANGELOG.