Update root README.md after last changes + create separate README for docs and future fmf sub-modules

[im-konge]

The current README.md is not accurate after last PRs:

• the generator is not delivered as jar but as Maven plugin
• it should contain configuration of the plugin
• there is TODO for the overview
• it misses @SuiteDoc annotation
• Makefiles are not part of the repository

Also, I think we should separate the README.md into multiple files, where in root of this repository will be simple overview with links to the other files. And then each README.md for particular generator - currently there is just documentation generator, but in case that we will add fmf generator, it should be in separate module, thus separate README.md