Extend the README

[deemp]

@yegor256 suggested to extend the README (link) with:

• A short overview of the project
• A quick start

[yegor256]

@deemp please, don't forget to remove this information from the statically generated site, to avoid duplication

[deemp]

@yegor256, good point. I'm not sure we should avoid duplication here. Some people may prefer to read the site instead of the README. Hence, they may want to read the project overview and the quick start on the site.

I believe we can share some sections between README and the site. mdbook and mdsh (tools that we use) allow inlining files.

[yegor256]

@deemp sure, rendering it in both places is fine, as long as you don't duplicate the content inside the repo