Documentation: Special considerations for wasm32

[briansmith]

We should document:

• wasm32 is supported.
• wasm32 is especially difficult to deal with w.r.t. timing side channel protections, as this is controlled by the wasm32 compiler and runtime.
• wasm32-wasi is supported without any feature flags.
• wasm32-unknown-unknown is supported if ring's wasm32_unknown_unknown_js feature is enabled, in which case Crypto.getRandomValues() will be used. We are open to accepting PRs to add to similar feature flags for other wasm32-unknown-unknown environments.

Libraries that want to work similarly should consider providing a similar feature flag to the library that triggers the wasm32_unknown_unknown_js feature, like this:

#[features]
wasm32_unknown_unknown_js = ["ring/wasm32_unknown_unknown_js"];

Alternatively, the application can always ask for the feature to be enabled itself, even if it doesn't otherwise directly use ring:

[dependencies]
ring = { version = "0.17.7", features = ["wasm32_unknown_unknown_js"] }