Document the proptest feature

Author: mbrubeck

README.md

@@ -31,6 +31,7 @@ The following optional features can be enabled in `Cargo.toml`:
 * `rand`: Adds implementations for various distribution types provided by the `rand` crate.
 * `serde`: Implements the `serde::Serialize` and `serde::Deserialize` traits.
 * `schemars`: Implements the `schemars::JsonSchema` trait.
+* `proptest`: Implements the `proptest::Arbitrary` trait.
 
 ## License