Provide overview about project components (#121)

* Add descriptions for folder structure + node

* Beef up descriptions of project components

* Fix list formatting

* Add note about nightly Rust

* Update README.md

Co-authored-by: Tomasz Drwięga <[email protected]>

* Update README.md

Co-authored-by: Tomasz Drwięga <[email protected]>

* Reword Tomek's currency-exchange suggestion

* Add link to Substrate's installation guide

Co-authored-by: Tomasz Drwięga <[email protected]>