doc: how to run example, for n00bs

[coolaj86]

Re: https://github.com/servo/html5ever/pull/475

1. Make sure your rustc version is 1.61.0 or greater

   rustup update
   rustup override unset
   rustup default stable
   rustc --version
   

2. Create a new project

   mkdir -p ./my-print-rcdom/
   pushd ./my-print-rcdom/
   cargo init
   

3. Update Cargo.toml to include the dependencies (also, 2018 appears to be the required edition)

   [package]
   name = "example01"
   version = "0.1.0"
   authors = ["Joe Doe <[email protected]>"]
   edition = "2018"
   
   # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
   
   [dependencies]
   html5ever = "0.25.2"
   markup5ever_rcdom = "0.1.0"
   

4. Replace src/main.rs with the contents of, for example, ./rcdom/examples/print-rcdom.rs

   rm ./src/main.rs
   curl -L -o ./src/main.rs https://raw.githubusercontent.com/servo/html5ever/1b34cf17c/rcdom/examples/print-rcdom.rs
   

5. Install, build, run (with input):

   cargo build
   curl https://example.com | cargo run
   

Troubleshooting

Weird bug:

warning: unused import: `html5ever::tendril::TendrilSink`
  --> src/main.rs:20:5
   |
20 | use html5ever::tendril::TendrilSink;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.

Changing the name field in Cargo.toml (to anything at all), saving, and running cargo install again.

A room full of rust developers, including one of the most popular teachers on udemy in the room here with us, cannot figure this out.