document that we support 3 months of nightlies

README.md

@@ -25,8 +25,9 @@ and then run the following in your project:
 cargo +nightly careful test
 ```
 
-Running `cargo careful` requires a recent nightly toolchain. You can also `cargo +nightly careful
-run` to execute a binary crate. All `cargo test` and `cargo run` flags are supported.
+Running `cargo careful` requires a recent nightly toolchain (the last 3 months of nightly versions
+are supported). You can also `cargo +nightly careful run` to execute a binary crate. All `cargo
+test` and `cargo run` flags are supported.
 
 The first time you run `cargo careful`, it needs to run some setup steps, which requires the
 `rustc-src` rustup component -- the tool will offer to install it for you if needed.