Expand documentation

[tarcieri]

Adds more documentation about the relationship with generic-array.

Also adds better toplevel documentation to the Array type, as well as some initial documentation on AsRef conversions and how to write generic code which presents a const generic API but converts to Array.

[newpavlov]

I think it's worth to move all crate-level docs to README. Right now we have a somewhat strange order of sections, first licensing information and then usage examples.

[tarcieri]

I think it's worth to move all crate-level docs to README.

The problem with that is you can't easily link to types using e.g. Array (with backticks), since when GitHub renders it the link will be dead. So generally I've put any documentation that makes extensive use of that sort of cross-linking/referencing in the rustdoc instead.