Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+
+**Thank your for opening a PR!**
+
+Before you proceed, please check the following notes.
+
+* Make sure you describe the changes introduced in the PR.
+* If this PR addresses an existing issue, please mention it here.
+* Check our [Guidelines for Contributing](https://github.com/bambinos/bambi/blob/main/CONTRIBUTING.md).
+* In particular, pay attention to the [pull request checklist](https://github.com/bambinos/bambi/blob/main/CONTRIBUTING.md#pull-request-checklist).
+* The most important points are to
+    + Install all the needed requirements (`requirements.txt`, `requirements-dev.txt`, etc.)
+    + Make sure all test pass.
+    + Make sure your code passes `black`.
+    + Make sure your code passes `pylint`.