Indicate that libm feature only builds on latest Rust

README.md

@@ -34,7 +34,8 @@ default-features = false
 # features = ["libm"]    # <--- Uncomment if you wish to use `Float` and `Real` without `std`
 ```
 
-The `Float` and `Real` traits are only available when either `std` or `libm` is enabled.
+The `Float` and `Real` traits are only available when either `std` or `libm` is enabled.  
+The `libm` feature is only available with Rust 1.31 and later.
 
 The `FloatCore` trait is always available.  `MulAdd` and `MulAddAssign` for `f32`
 and `f64` also require `std` or `libm`, as do implementations of signed and floating-