Write a plugin development guide

[fdiblen]

There is no documentation on how to create a new indicator plugin. The README mentions the plugin architecture briefly, but someone who wants to add a new check has to reverse-engineer the existing plugins to figure that out.

A step-by-step guide with a skeleton example would lower the barrier to contribution significantly. Something like:

1. Create a new file in src/resqui/plugins/
2. Subclass IndicatorPlugin
3. Set the class attributes (name, version, id, indicators)
4. Implement your indicator methods
5. Register in __init__.py
6. Add to configuration files
7. Test it

[vuillaut]

I think this is a priority to increase the number of plugins and indicators.