Document how to add functionality and modules to the entry points

[ns-rse]

This should be well documented to make it easy to extend. Broadly the steps involved are...

1. Write a new module under topostats/<module>.py
2. Add configuration options to topostats/default_config.yaml
3. Validate the configuration options in topostats/validation.py
4. Add a sub-parser to topostats/entry_point.py
5. Add a function to process (ideally in parallel) to topostats/run_modules.py

Will bulk this out as I go through the process of writing these.