docs: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+<!--  Thank you for sending a pull request! Please take a moment to review the following:
+
+- If this is your first contribution, please read our guide for new contributors https://github.com/openpgpjs/openpgpjs/blob/main/NEW-CONTRIBUTORS.md and our general contributing guide https://github.com/openpgpjs/openpgpjs/blob/main/CONTRIBUTING.md
+- Make sure that you have added or run the appropriate tests https://github.com/openpgpjs/openpgpjs/blob/main/CONTRIBUTING.md#testing
+- If your PR is not finished, please mark it as a draft
+-->
+
+**Description:**
+<!-- Explain what this PR does and why we need it  -->
+
+**Which issue(s) this PR fixes:**
+<!--
+*Automatically closes linked issue when PR is merged.
+Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
+-->
+Fixes #
+
+**Breaking changes**
+<!-- Does this PR introduce any breaking changes? If so, list which ones -->