Add Pull Request template file to the MariaDB/server repository

.github/pull_request_template.md

+<!--
+Thank you for contributing to the MariaDB Server repository!
+
+You can help us review your changes faster by filling this template <3
+
+If you have any questions related to MariaDB or you just want to
+hang out and meet other community members, please join us on
+https://mariadb.zulipchat.com/ .
+-->
+
+<!--
+If you've already identified a https://jira.mariadb.org/ issue
+that seems to track this bug/feature, please add its number below.
+-->
+- [x] *The Jira issue number for this PR is: MDEV-_____*
+
+<!--
+An amazing description should answer some questions like:
+1. What problem is the patch trying to solve?
+2. If some output changed, what was it looking like before
+   the change and how it's looking with this patch applied
+3. Do you think this patch might introduce side-effects in
+   other parts of the server?
+-->
+## Description
+TODO: fill description here
+
+## How can this PR be tested?
+TODO: fill steps to reproduce here, if applicable,
+      or remove the section
+
+<!--
+Tick one of the following boxes [x] to help us understand
+if the base branch for the PR is correct
+-->
+## Basing the PR against the correct MariaDB version
+- [ ] *This is a new feature and the PR is based against the latest MariaDB development branch*
+- [ ] *This is a bug fix and the PR is based against the earliest branch in which the bug can be reproduced*
+
+<!--
+You might consider answering some questions like:
+1. Does this affect the on-disk format used by MariaDB?
+2. Does this change any behavior experienced by a user
+   who upgrades from a version prior to this patch?
+3. Would a user be able to start MariaDB on a datadir
+   created prior to your fix?
+-->
+## Backward compatibility
+TODO: fill details here, if applicable, or remove the section
+