Document `gitoid` crate goals.

[alilleybrinker]

The gitoid crate has some challenging and precise goals which inform the direction we're planning to take it, and which ought to be documented somewhere (probably, for convenience, in the API Docs).

They include:

• Minimize the size of the GitOid struct being stored in memory.
• Permit as clean and easy C interop as possible, so that the code here can be used as the basis for implementations in other languages. Ideally achieve this by making the GitOid type #[repr(C)] so the fields can be accessed by C code directly.
• Support custom hashes (anything that implements the Digest trait really).
• Support use in embedded environments, meaning no_std support in Rust, with optional use of alloc or std, progressively enhancing the capabilities of the base library.

This issue is to track getting these things documented in the gitoid crate documentation.

[alilleybrinker]

Thinking about this more; the more important goals are the ones now spelled out in the README. I am closing this in favor of those.