Updated README

Supported only on Rust Nightly due to unstable features
diff --git a/README.md b/README.md
index 629ac5d..b95954a 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 [Documentation](https://kkawakam.github.io/rustyline)
 
 ## Build
-This project uses Cargo and Rust Stable
+This project uses Cargo and Rust Nightly
 ```bash
 cargo build --release
 ```