Add MSRV in the README.md

This adds an MSRV badge in the `README.md` similar to the [`regex` project](https://github.com/rust-lang/regex/blob/master/README.md). I got the MSRV from the `CHANGELOG.md`
rust-lang · Sep 13, 2020 · 35f7b47 · 35f7b47
1 parent d26b37a
commit 35f7b47
Showing 1 changed file with 1 addition and 0 deletions.
diff --git a/README.md b/README.md
@@ -4,6 +4,7 @@ hashbrown
 [![Build Status](https://travis-ci.com/rust-lang/hashbrown.svg?branch=master)](https://travis-ci.com/rust-lang/hashbrown)
 [![Crates.io](https://img.shields.io/crates/v/hashbrown.svg)](https://crates.io/crates/hashbrown)
 [![Documentation](https://docs.rs/hashbrown/badge.svg)](https://docs.rs/hashbrown)
+[![Rust](https://img.shields.io/badge/rust-1.36.0%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/hashbrown)
 
 This crate is a Rust port of Google's high-performance [SwissTable] hash
 map, adapted to make it a drop-in replacement for Rust's standard `HashMap`