Update pull_request_template.md
Makes it easy to understand what the person is meaning.
@@ -3,5 +3,6 @@ Please ensure that your pull request meets the following requirements - thanks !
* Follows our existing [codestyle](https://github.com/k9mail/k-9/wiki/CodeStyle).
* Does not break any unit tests.
* Contains a reference to the issue that it fixes.
+* On cosmetic changes add one or multiple images, if possible.
