Added helpful notice regarding proptest versions.
//! trying to find a sequential ordering that results
//! in the same return values.
//!
+//! **important**: the crate makes use of
+//! [proptest](https://crates.io/crates/proptest) via
+//! macros. ensure that you are using the same version
+//! of `proptest` that `model` lists in `Cargo.toml`,
+//! otherwise mismatched API change will manifest as
+//! strange compile-time errors hidden in macros.
+//!
//! model-based testing:
//!
//! ```rust,noexecute
