Write pallet_evm README (#7487)

* describe pallet evm

* add license

* write readme into lib.rs

* change some sentence

Co-authored-by: Wei Tang <[email protected]>

* update implementation status

Co-authored-by: Wei Tang <[email protected]>

* update readme according to review

* Update frame/evm/src/lib.rs

* fix line length

Co-authored-by: Wei Tang <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>