Reference fixed-capacity-vec in your "Comparison" docs

[ijackson]

Hi. I saw your crate in TWIR. It looks like it fills a niche :-).

I noticed you have a "Comparison" section. I was wondering if you'd welcome an MR to your docs to link to fixed-capacity-vec. (Or would like to write such a link yourself.)

Certainly it seems like at least your crate or mine ought to have a discussion of heap-allocation-based types (like Vec, FixedCapacityVec) vs slice-backed types (with lifetime, eg your FixedSliceVec) vs on-stack types (ArrayVec).

LMK what you think. I'm happy to do wordsmithing etc.