GitHub setup: PR template or development as default branch

[altendky]

With main as the default branch, PRs are automatically made against this. If this is kept then a PR template should likely be added to remind people as they are creating the PR that it should be developed and submitted against development. If I understand the branch usage intention correctly...

Alternatively, the branch which should be developed against could be made the default. I'm guessing the interest in main being the default is so that people that aren't developing can clone and jump straight to running a 'stable' version. Following #61 where plotman becomes installable we can start publishing releases to PyPI and people that aren't developing can skip the git clone and the source entirely.