* Explicitly state on the PR template that we can squash commits
This way we don't need to ask every time, and users who for some reason
would not like us to squash their commits can explicitly state so.
This creates a separate section from 'features' for small changes which
don't usually require user intervention, such as:
* Human readable session duration
* New junitxml fields
* Improved colors in terminal
* etc.
The idea is to better match user expectations about new actual
features in the "Features" section of the changelog.
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.