Add documentation to the README.md

[vepanimas]

We need to describe in the README the most important aspects of configuring and using the plugin. Providing documentation on a separate website or in the other repo is not very discoverable.

Sections to add:

1. Configuration files, their syntax, possible project layouts for different scenarios.
2. Environment variables.
3. Schema introspection.
4. Writing and executing queries.
5. Using Scratch files for writing queries (especially comment-based schema configuration in them).
6. Frameworks support.
7. FAQ