DOC: Rust version in docs

Required rust version should be in rustdoc

Reason: Written in one or a few places. Information is versioned with
the crate on docs.rs

src/lib.rs

@@ -5,7 +5,6 @@
 //!
 //! - `std`
 //!   - Optional, enabled by default
-//!   - Requires Rust 1.6 *to disable*
 //!   - Use libstd
 //!
 //! - `use_union`
@@ -14,6 +13,8 @@
 //!   - Use the unstable feature untagged unions for the internal implementation,
 //!     which has reduced space overhead
 //!
+//! **arrayvec Requires Rust 1.18**
+//!
 #![doc(html_root_url="https://docs.rs/arrayvec/0.3/")]
 #![cfg_attr(not(feature="std"), no_std)]
 extern crate odds;