create short developer faq for new contributors

As title, guide should explain how to do common tasks including:
- add new config options to admin page
- how to handle individual config options
- how to add new tests
- general design principles

Start by locating the plugin's admin page configuration handling and existing tests. Document how to add and handle config options, add tests, and follow the project's general design principles; done means a concise developer FAQ covers all four requested topics.

Written by the indexing model from the issue text.