Document crate Cargo features

[edmorley]

Hi

The zstd crate currently has the legacy, arrays, zdict_builder features enabled by default:

zstd-rs/Cargo.toml

Lines 31 to 32 in e470f00

	[features]
	default = ["legacy", "arrays", "zdict_builder"]

I was curious what impact using default-features = false would have, however, could not find any docs on those default features (or most of the optional features) on either of:
https://docs.rs/zstd/latest/zstd/
https://github.com/gyscos/zstd-rs/blob/main/Readme.md

For the features that are passed to zstd-safe, I also tried checking here for feature docs:
https://docs.rs/zstd-safe/latest/zstd_safe/
https://github.com/gyscos/zstd-rs/blob/main/zstd-safe/Readme.md

There are some inline code comments in zstd-sys's Cargo.toml:

zstd-rs/zstd-safe/zstd-sys/Cargo.toml

Lines 68 to 78 in e470f00

	debug = [] # Enable zstd debug logs
	experimental = [] # Expose experimental ZSTD API
	legacy = [] # Enable legacy ZSTD support (for versions < zstd-0.8)
	non-cargo = [] # Silence cargo-specific build flags
	pkg-config = [] # Use pkg-config to build the zstd C library.
	std = [] # Deprecated: we never use types from std.
	zstdmt = [] # Enable multi-thread support (with pthread)
	thin = [] # Optimize binary by size
	no_asm = [] # Disable ASM files (only on amd64 for decompression)
	zdict_builder = [] # Enable dictionary building (dictionary _using_ is always supported).
	no_wasm_shim = [] # Disable wasm shims (in case your wasm toolchain includes a C stdlib).

...however, these are quite brief, don't cover all of the features present in the top-level zstd crate, and were less obvious to find.

Some docs on the module rustdocs and/or the README would help with discoverability :-)

[poliorcetics]

I concur, documentation would be nice

It's possible to document the features in the Cargo.toml and then use https://docs.rs/document-features/latest/document_features/ to make that appear in the crate docs, which helps ensure the feature docs are up to date by being just above the relevant feature declaration