Added mention of `i128` feature to ‘README.md’

README.md

@@ -25,7 +25,7 @@ extern crate num_traits;
 ## Features
 
 This crate can be used without the standard library (`#![no_std]`) by disabling
-the default `std` feature.  Use this in `Cargo.toml`:
+the default `std` feature. Use this in `Cargo.toml`:
 
 ```toml
 [dependencies.num-traits]
@@ -36,6 +36,8 @@ default-features = false
 The `Float` and `Real` traits are only available when `std` is enabled. The
 `FloatCore` trait is always available.
 
+Implementations for `i128` and `u128` are only available when `i128` is enabled.
+
 ## Releases
 
 Release notes are available in [RELEASES.md](RELEASES.md).