Readme improvement

[viktorprogger]

Let's add information about available keys in composer.json and configuration.php. I'd like to have this information before the Config Groups block. It will describe the top-level configuration options and give a definition about what "config group" is.

[samdark]

• https://github.com/yiisoft/docs/blob/master/022-config-groups.md
• https://github.com/yiisoft/docs/blob/master/guide/en/concept/configuration.md

[viktorprogger]

@samdark I'm about config-plugin-options, config-plugin keys, and so on. It's too difficult now to understand where to define config groups.