Document memory behaviour

I just read your [blog post](https://jam1.re/blog/binread-a-declarative-rust-binary-parsing-library) and bit of the docs. This crate looks really cool!

Can you document a bit about the memory behaviour of the generated code? For example, you mention nom in your blogpost, and how it has this "zero-copy" property, which is made more concrete in the README of nom as:
> if a parser returns a subset of its input data, it will return a slice of that input, without copying

I suppose you cannot make the exact same guarantees, especially given the escape hatch of `parse_with`, but I imagine binread will typically have a similar zero-copy property, right?

The issue mentions the generated parsing code, the README discussion of zero-copy behavior, and the parse_with escape hatch, but no repository file or test. Start by tracing how generated code handles input and parse_with, then document the observed memory guarantees and limitations with a clear completion criterion.

Written by the indexing model from the issue text.