[snippy][NFC] handle missed error code (#323)

Errc::NoElements was recently added but we forgot to handle it in
SnippyErrorCategory::message

Co-authored-by: Alexander Romanov <[email protected]>

Adding documentation and examples (#274)

Co-authored-by: Konstantin Vladimirov <[email protected]>

[snippy] Update ways to contribute in readme (#7)