docs: cargo install cargo expand: Recommend --version

Fixes eupn#90.

src/lib.rs

@@ -74,6 +74,13 @@
 //! cargo install --locked cargo-expand
 //! ```
 //!
+//! (In CI, you'll want to pin to a particular version,
+//! since
+//! [cargo expand's output is not stable across versions](https://github.com/dtolnay/cargo-expand/issues/179).
+//! Look up the
+//! [current version](https://crates.io/crates/cargo-expand)
+//! and do something like `cargo install --locked --version 1.0.81 cargo-expand`.)
+//!
 //! ## Setting up a test project
 //!
 //! In your crate that provides procedural or declarative macros, under the `tests` directory,