feat: add PR template

Author: juliapampus

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,17 @@
+## What this PR changes/adds
+
+_Briefly describe WHAT your PR changes, which features it adds/modifies._
+
+## Why it does that
+
+_Briefly state why the change was necessary._
+
+## Further notes
+
+_List other areas of the documents that have changed but are not necessarily linked to the main feature. This could be editorial changes or mistakes in example files that were encountered and were fixed inline, etc._
+
+## Linked Issue(s)
+
+Closes # <-- _insert issue number if one exists_
+
+_Please be sure to take a look at the [contributing guidelines](https://github.com/eclipse-dataspace-protocol-base/DataspaceProtocol/blob/main/CONTRIBUTING.md) and our [etiquette for pull requests](https://github.com/eclipse-dataspace-protocol-base/DataspaceProtocol/blob/main/PR_ETIQUETTE.md)._