Add checklist for PRs (#1601)

Co-authored-by: jschaul <jschaul@users.noreply.github.com>

.github/pull_request_template.md

@@ -0,0 +1,11 @@
+## Checklist
+
+ - [ ] Title of this PR explains the impact of the change.
+ - [ ] The description provides context as to why the change should occur and
+       what the code contributes to that effect. This could also be a link to a
+       JIRA ticket or a Github issue, if there is one.
+ - [ ] The CHANGELOG.md file in the *Unreleased* section has been updated to
+       explain the change which will be included in the release notes.
+ - [ ] If a component uses a new or changed internal endpoint of another
+       component, this is mentioned in the CHANGELOG.md.
+- [ ] If this PR creates a new endpoint, or adds a new configuration flag, the endpoint / config-flag checklist (see Wire-employee only backend [wiki page](https://github.com/zinfra/backend-wiki/wiki/Checklists)) has been followed.