The README.md isn't part of the rust code, so it's not checked by this unless you use tools to generate your README from doc comments like https://github.com/livioribeiro/cargo-readme.
Actually, it's possible and even frequently used, for example in lazy-static, through the `doc_comment` crate.
#[cfg(doctest)]
#[macro_use]
extern crate doc_comment;
#[cfg(doctest)]
doctest!("../README.md");
https://github.com/rust-lang-nursery/lazy-static.rs/blob/mas...
Code examples from the documentation automatically become "documentation tests" to make sure your examples are still up to date when updating your code.
[1] https://doc.rust-lang.org/rustdoc/documentation-tests.html