Add notes about testing and deps to README.md

Author: rklaehn

README.md

@@ -3,6 +3,10 @@
 This crate provides a data structure with identical behaviour to a `BTreeMap<u64, T>`,
 but optimized for the case where keys are mostly consecutive.
 
+It has no dependencies and should work in all environments.
+
+Tests are comparing against `BTreeMap<u64, T>` using [proptest].
+
 ## License
 
 Copyright 2025 N0, INC.
@@ -19,3 +23,5 @@ at your option.
 ## Contribution
 
 Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.
+
+- proptest: https://docs.rs/proptest/latest/proptest/