This streamlines the PR template text and adds a more in-depth explanation
about how the changelog entries work because this topic is a common source of
confusion:
- How to name the files.
- Which formatting to use (people in general assume it is Markdown).
- Recommend adding `.rst` extension to changelog files to help with the
above (`towncrier` doesn't care).
This was heavily inspired by the excellent python-trio/trio docs.
Bruno Oliveira