Add a min-rustc badge and document compatibility

Author: cuviper

README.md

@@ -2,6 +2,7 @@
 
 [![crate](https://img.shields.io/crates/v/num-integer.svg)](https://crates.io/crates/num-integer)
 [![documentation](https://docs.rs/num-integer/badge.svg)](https://docs.rs/num-integer)
+![minimum rustc 1.8](https://img.shields.io/badge/rustc-1.8+-red.svg)
 [![Travis status](https://travis-ci.org/rust-num/num-integer.svg?branch=master)](https://travis-ci.org/rust-num/num-integer)
 
 `Integer` trait and functions for Rust.

src/lib.rs

@@ -9,6 +9,10 @@
 // except according to those terms.
 
 //! Integer trait and functions.
+//!
+//! ## Compatibility
+//!
+//! The `num-integer` crate is tested for rustc 1.8 and greater.
 
 #![doc(html_root_url = "https://docs.rs/num-integer/0.1")]