Add pull request template for contributions

Add a pull request template to guide contributors.

Author: paolosalvatori

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,28 @@
+<!--
+Please refer to the contribution guidelines before raising a PR.
+https://github.com/localstack/localstack/blob/main/docs/CONTRIBUTING.md
+-->
+
+## Motivation
+
+<!--
+Elaborate the background and intent for raising this PR.
+-->
+
+## Changes
+
+<!--
+Summarise the changes proposed in the PR.
+-->
+
+## Tests
+
+<!--
+Optional: How are the proposed changes tested?
+-->
+
+## Related
+
+<!--
+Optional: Links to related issues and references (e.g., Linear IDs).
+-->