ci: improve PR template

Author: chrisgrieser

.github/pull_request_template.md

@@ -0,0 +1,9 @@
+## What problem does this PR solve?
+
+## How does the PR solve it?
+
+## Checklist
+- [ ] Used only `camelCase` variable names.
+- [ ] If functionality is added or modified, also made respective changes to the
+  `README.md` (the `.txt` file is auto-generated and does not need to be
+  modified).