Update documentation

README.md

@@ -30,6 +30,7 @@ It's possible to generalize these if done safely, but the implementation doesn't
 
 The implementation keeps dependencies to a minimum, and is `no_std` right out of the box.
 You can enable the optional `serde` feature for proof (de)serialization support.
+You can enable the optional `std` feature for corresponding dependency features.
 
 ## Security