Document the `playground` Cargo metadata

[dns2utf8]

Hey team

I made a test with the shiny new tool: https://play.rust-lang.org/?gist=118112c0ca3014d3683dfd6db7fa99ef&version=stable

Can you enable some the features for the crate?

[shepmaster]

Can you enable some the features for the crate?

No, we do not wish to have anything to do with how any particular crate is exposed. Instead, you can add the playground feature to the libraries Cargo.toml. Here's how Serde does it.

note that this method no longer works. You want to use #192 (comment) instead

[shepmaster]

I'm going to repurpose this issue for adding documentation about this feature.

[shepmaster]

From #326

This is based on the custom metadata configurable for docs.rs -- see https://docs.rs/about. Ours looks like:

# Cargo.toml
[package]
name = "test"

[package.metadata.playground]
default-features = true
features = ["std", "extra-traits"]
all-features = false