Updated PR template to add changelog checkbox (#1293)

Signed-off-by: Madeline <madeline.murray@consensys.net>

.github/pull_request_template.md

@@ -6,3 +6,7 @@
 ## Fixed Issue(s)
 <!-- Please link to fixed issue(s) here using format: fixes #<issue number> -->
 <!-- Example: "fixes #2" -->
+
+## Changelog
+
+- [ ] I thought about the changelog and included a [changelog update if required](https://wiki.hyperledger.org/display/BESU/Changelog).