Mention feature unification?

Binary crates can be built with or without feature unification. Building without sometimes results in a smaller artefact.

I'm unsure if this is worth mentioning, but I found no issues or pull requests mentioning it so I thought it may be worthwhile to bring up at least once. If it is, then I would be happy to write something up.

Examples:
```sh
# Unification enabled:
cargo build
cargo build --bin my-crate
```
```sh
# Unification disabled:
cd my-crate && cargo build
cargo build --package my-crate
```