add pr template (#137)

Author: MarcoGorelli

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+# Thank you for opening a pull request!
+
+Please check our [style guide](https://github.com/pymc-devs/pymc3/wiki/PyMC's-Jupyter-Notebook-Style), and also make sure that the notebooks you've modified pass the `pre-commit` checks. If, for example, you modified `notebook1.ipynb` and `notebook2.ipynb`, you could do this by running:
+
+```bash
+pre-commit run --files notebook1.ipynb notebook2.ipynb
+```
+
+You may need to run it a couple of times before all the checks pass.