Add GitHub template for Pull Requests (#10)

Author: FrancescoCasalegno

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,22 @@
+Fixes #{issue-id-number}.
+
+## Description
+
+Please provide here a summary of the changes introduced by this PR.
+
+## How to test?
+
+Please provide here instructions on how to test the changes introduced by this PR.
+(if some changes cannot be tested by automated tests)
+
+## Checklist
+
+- [ ] This PR refers to an issue from the [issue tracker](https://github.com/BlueBrain/morphoclass/issues).
+  (if it is not the case, please create an issue first).
+- [ ] Unit tests added.
+  (if needed)
+- [ ] `setup.cfg`, `requirements.txt`, and `constraints.txt` updated with new dependencies.
+  (if needed)
+- [ ] Type annotations added.
+  (if a function is added or modified)
+- [ ] All CI tests pass.