Add link to crates.io in README

Author: Saluki

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://saluki.semaphoreci.com/badges/arp-scan-rs/branches/master.svg?style=shields)](https://saluki.semaphoreci.com/projects/arp-scan-rs)
 [![dependency status](https://deps.rs/repo/github/Saluki/arp-scan-rs/status.svg)](https://deps.rs/repo/github/Saluki/arp-scan-rs)
+![crates.io](https://img.shields.io/crates/v/arp-scan.svg)
 
 Find all hosts in your local network using this fast ARP scanner. The CLI is written in Rust and provides a minimal scanner that finds all hosts using the ARP protocol. Inspired by the awesome [arp-scan project](https://github.com/royhills/arp-scan).