Overhaul example with more information and CI

[djhoese]

As part of my process to complete pytroll/satpy#2138 I'm updating this repository with more information. In this PR I've:

1. Added more information to the README on how to install the package, what the contents are, how to contribute, and where the Satpy plugin documentation is.
2. Added GitHub Actions CI
3. Added a .gitignore
4. Added more standard information to the setup.py for now. See below.

TODO

I still plan on replacing the setup.py with a pyproject.toml.

[djhoese]

CI passes now: https://github.com/djhoese/satpy-composites-plugin-example/actions

[djhoese]

I switched to pyproject.toml completely but it turns out pip/setuptools don't work with editable installs so the tests fail. At least, they will until pypa/setuptools#2816 is resolved.

[mraspaud]

Some comments about the licencing, otherwise looks good, thanks for the cleanup!