chore: add template for pull requests

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -0,0 +1,19 @@
+## Checklist for PR
+
+[ ] PR is only about one and only one concern
+[ ] Description contains a short title, an optional description, and the sections BEFORE and AFTER
+[ ] (A) new short test(s) show(s) that the PR is working
+
+## Some more infos
+
+- Short PR: adding two features and formatting code in other features is too much
+
+- BEFARE and AFTER sections:
+
+  ```
+  BEFORE:
+      feature did this.
+
+  AFTER:
+      now, the features does that.
+  ```