[hibernate#1946] Add a simple PR template

Author: marko-bekhta

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!--
+If this is your first time contributing to the project, 
+please consider reviewing https://github.com/hibernate/hibernate-reactive/blob/main/README.md
+
+Please include a link to the GitHub issue solved by this PR in the description;
+see https://github.com/hibernate/hibernate-reactive/issues.
+-->
+
+[Please describe here what your change is about]
+
+<!--
+Please read and do not remove the following lines:
+-->
+----------------------
+By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license](https://www.apache.org/licenses/LICENSE-2.0.txt).
+For more information on licensing, please check [here](https://github.com/hibernate/hibernate-reactive/blob/main/LICENSE).
+
+----------------------