Add updating CHANGELOG.md to MR template

.gitlab/merge_request_templates/DEFAULT.md

 Thank you for making a Merge Request. Before you merge this, please make sure
 that you:
 
-- [ ] Updated chart version number
+- [ ] Updated chart version number in `Chart.yaml`
+- [ ] Updated the `CHANGELOG.md`
 - [ ] Manually tested chart changes with the stackspin bootstrap script
 - [ ] Created an MR in stackspin/stackspin that applies the changes from this MR
 - [ ] Tests in that MR in stackspin/stackspin succeed