Fix #540

Changes proposed in this pull request:

• Modify PR template to precise to add assert message and link to example docstring
• Update CONTRIBUTING to add a "Write test for your code" section before the "Run tests locally" one

The following steps were realized, as well (if applies):

• Write docstrings to your code
• For new functionalities: Explain in readthedocs
• Update the CHANGELOG.md

_{For more information on how to contribute check the CONTRIBUTING.md.}