Add pull request template

Author: hynek

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+# Pull Request Check List
+
+This is just a friendly reminder about the most common mistakes.  Please make sure that you tick all boxes.  But please read our [contribution guide](https://www.structlog.org/en/latest/contributing.html) at least once, it will save you unnecessary review cycles!
+
+If an item doesn't apply to your pull request, **check it anyway** to make it apparent that there's nothing left to do.
+
+- [ ] Added **tests** for changed code.
+- [ ] Updated **documentation** for changed code.
+    - [ ] New functions/classes have to be added to `docs/api.rst` by hand.
+    - [ ] Changed/added classes/methods/functions have appropriate `versionadded`, `versionchanged`, or `deprecated` [directives](http://www.sphinx-doc.org/en/stable/markup/para.html#directive-versionadded).  Find the appropriate next version in our [``__init__.py``](https://github.com/hynek/structlog/blob/master/src/structlog/__init__.py) file.
+- [ ] Documentation in `.rst` files is written using [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/).
+- [ ] Changes (and possible deprecations) are documented in the [changelog](https://github.com/hynek/structlog/blob/master/CHANGELOG.rst).
+
+If you have *any* questions to *any* of the points above, just **submit and ask**!  This checklist is here to *help* you, not to deter you from contributing!