Attribute and doc comment ordering

Closes #72

Author: Nick Cameron

guide/guide.md

@@ -81,6 +81,7 @@ Prefer line comments (`///`) to block comments (`//* ... */`).
 Prefer outer doc comments (`///` or `//*`), only use inner doc comments (`//!`
 and `//*!`) to write module-level or crate-level documentation.
 
+Doc comments should come before attributes.
 
 ### Attributes
 
@@ -100,6 +101,11 @@ struct CRepr {
 }
 ```
 
+There must only be a single `derive` attribute. Note for tool authors: if
+combining multiple `derive` attributes into a single attribute, the ordering of
+the derived names should be preserved. E.g., `#[derive(bar)] #[derive(foo)]
+struct Baz;` should be formatted to `#[derive(bar, foo)] struct Baz;`.
+
 ## [Non-formatting conventions](advice.md)
 
 ## [Cargo.toml conventions](cargo.md)