Add rustc version badge to readme

Author: taiki-e

README.md

@@ -14,6 +14,10 @@
   <a href="https://crates.io/crates/futures-preview">
     <img alt="Crates.io" src="https://img.shields.io/crates/v/futures-preview.svg">
   </a>
+
+  <a href="https://blog.rust-lang.org/2019/07/04/Rust-1.36.0.html">
+    <img alt="Rustc Version" src="https://img.shields.io/badge/rustc-1.36+-lightgray.svg">
+  </a>
 </p>
 
 <p align="center">