Document encode_bound, decode_bound and encode_bound custom attributes

[emostov]

#279 Adds the mel_bound derive. AFAICT it is not mentioned in the top level docs. It would be nice if it was mentioned in the top level docs for easy discoverability. (Maybe in the Derive attributes section?)

cc @KiChjang

[KiChjang]

For clarity, it's this part of the documentation: https://docs.rs/parity-scale-codec/2.3.0/parity_scale_codec/#derive-attributes

[wischli]

Would be great to have some documentation. I am currently upgrading all pallets to be bound in storage size and stumbled upon this derive which seems to be necessary in some cases. Would love to understand how to use it properly. In the Substrate repo it is mostly used without input.