Include html_root_url attribute

[dtolnay]

This is important when running cargo doc --no-deps for a crate that depends on reqwest. Without html_root_url, there will not be links to any of reqwest's types. With html_root_url all of reqwest's types will be linked relative to that url.

#![doc(html_root_url = "https://docs.rs/reqwest//0.6.2")]

It may be good to add a reminder in Cargo.toml so this doesn't fall out of date. Similar to serde-rs/serde@dc7ab26.