diff --git a/README.md b/README.md
index 2caf54b..1705b92 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,8 @@ This crate provides fast functions for printing integer primitives to an
 
 See also [`dtoa`] for printing floating point primitives.
 
+*Version requirement: rustc 1.0+*
+
 [`io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
 [`fmt::Write`]: https://doc.rust-lang.org/core/fmt/trait.Write.html
 [libcore]: https://github.com/rust-lang/rust/blob/b8214dc6c6fc20d0a660fb5700dca9ebf51ebe89/src/libcore/fmt/num.rs#L201-L254