Adds changelog in pull request template

Author: fakufaku

PULL_REQUEST_TEMPLATE.md

@@ -6,5 +6,6 @@ Please also refer to the doc on [contributing](https://pyroomacoustics.readthedo
 - [ ] Have you run the tests by doing `nosetests` or `py.test` at the root of the repo ?
 - [ ] Have you checked that the doc builds properly and that any new file has been added to the repo ? How to do that is covered in the [documentation](https://pyroomacoustics.readthedocs.io/en/pypi-release/contributing.html#documentation).
 - [ ] Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
+- [ ] Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".
 
 Happy PR :smiley: