update documentation

[folkertdev]

Mostly we can use newer rustdoc feature for more cross-linking. I also added docs on the feature flags in the readme.

[folkertdev]

I removed this because crates.io shows you what to add and cargo add exists now. The risk of this snippet getting out of sync is high.

[bjorn3]

This should probably be kept out until right before 0.5 is actually released to avoid confusing people who are looking at the readme on github rather than on crates.io.

[folkertdev]

sure, to refer back to this later, the proposed text for the release is

Features

By default, this crate will compile the libbz2 C library from source. That requires a functional C toolchain,
and may not work for all targets (in particular webassembly).

libbz2-rs-sys

Since version 0.5.0, this crate also supports using libbz2-rs-sys,
a drop-in compatible rust implementation of libbz2. With this feature enabled, cross-compilation should work
like any other rust code, and no C toolchain is needed to compile this crate or its dependencies.

bzip2 = { version = "0.5.0", default-features = false, features = ["libbz2-rs-sys"] }

static

Statically links libbz2. When libbz2-rs-sys is enabled, static mode is always used.