Improve check list (#5587)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,19 @@
-<!-- describe the changes you have made here: what, why, ... 
-     Link issues by using the following pattern: [#333](https://github.com/JabRef/jabref/issues/333) or [koppor#49](https://github.com/koppor/jabref/issues/47).
-     The title of the PR must not reference an issue, because GitHub does not support autolinking there. -->
+<!-- 
+Describe the changes you have made here: what, why, ... 
+Link issues by using the following pattern: #333.
+If you fixed a koppor issue, link it with following pattern: [koppor#47](https://github.com/koppor/jabref/issues/47).
+The title of the PR must not reference an issue, because GitHub does not support autolinking there.
+-->
 
 
-----
+<!-- 
+- All items with `[ ]` are still a TODO.
+- All items checked with `[x]` are done.
+- Remove items not applicable
+-->
 
-- [ ] Change in CHANGELOG.md described
-- [ ] Tests created for changes
-- [ ] Manually tested changed features in running JabRef
+- [ ] Change in CHANGELOG.md described (if applicable)
+- [ ] Tests created for changes (if applicable)
+- [ ] Manually tested changed features in running JabRef (always required)
 - [ ] Screenshots added in PR description (for bigger UI changes)
 - [ ] [Checked documentation](https://docs.jabref.org/): Is the information available and up to date? If not: Issue created at <https://github.com/JabRef/user-documentation/issues>.