Document markdown wrapping policy (#3680)

And add note to README.md about CONTRIBUTING.md existing.

CONTRIBUTING.md

@@ -22,7 +22,12 @@ to the branch" checkbox.
 We use unstable Rustfmt options across the project, so please run
 `cargo +nightly fmt` before submitting your work. If you are unable to do so,
 the maintainers can do it for you before merging, just state so in your pull
-request description.
+request description. For details on how to use nightly, consult [the
+documentation][toolchains].
+
+When editing markdown files (`.md`) they must be wrapped at 80 characters.
+
+[toolchains]: https://rust-lang.github.io/rustup/concepts/toolchains.html
 
 #### Handling review
 

README.md

@@ -33,6 +33,10 @@ Winit is designed to be a low-level brick in a hierarchy of libraries. Consequen
 show something on the window you need to use the platform-specific getters provided by winit, or
 another library.
 
+## CONTRIBUING
+
+For contributing guidelines see (CONTRIBUTING.md)[./CONTRIBUTING.md].
+
 ## MSRV Policy
 
 This crate's Minimum Supported Rust Version (MSRV) is **1.73**. Changes to