Document SCALE Optimizations for Integer Types and Vectors Thereof

[danforbes]

Document the concepts mentioned by @thiolliere in comments on #38.

[gui1117]

Indeed maybe mentionning those optimization for the rust implementation (and so the one used in FRAME) of parity-scale-codec is interesting.
Also I guess we should document them in the code as well.

[bkchr]

Okay, we speak here about the internal optimizations of the implementation? Why should they be documented here?

This is nothing a user should care about, this is just an optimization of the implementation.

[danforbes]

I tend to agree with @bkchr. I think it is very important to be careful that we don't document too much because it will just confuse our users. Internal optimizations definitely fall into the realm of "too much" for me.