chore(.github): adjust PULL_REQUEST_TEMPLATE

Mirrors bpmn-io/.github#7

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,20 @@
+### Proposed Changes
+
 <!--
+Add relevant context (issue fixed or related to), 
+a capture of the UI changes (if any) as well as 
+steps to try out your changes.
+--> 
+
+### Checklist
 
-Thanks for creating this pull request!
+To ensure you provided everything we need to look at your PR:
 
-Please make sure to link the issue you are closing as "Closes #issueNr". 
-This helps us to understand the context of this PR.
+* [ ] **Brief textual description** of the changes present
+* [ ] **Visual demo** attached
+* [ ] **Steps to try out** present, i.e. [using the `@bpmn-io/sr` tool](https://github.com/bpmn-io/sr)
+* [ ] Related issue linked via `Closes {LINK_TO_ISSUE}` or `Related to {LINK_TO_ISSUE}`
 
+<!--
+Thanks for creating this pull request! ❤️
 -->