Proposal: markdown description of the WASM support notes

[Maayan Hanin]

The inspiration for this proposal stems from:

1. The limited-functionality field
2. The way the readme field works in crate manifests (cargo.toml files), see: https://doc.rust-lang.org/cargo/reference/manifest.html#the-readme-field

I'm proposing we add a field similar to the [package] readme field that lets the author elaborate on their crate's WASM support in a markdown file.
Something like wasm-readme which the author would point to a markdown file in their repo.
If present, wasm-rs will interpret it as markdown and render it on the crate's page.

[Yurii Rashkovskii]

I like it.

[Yurii Rashkovskii]

Addressed in #6