Add latest version badge

README.md

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+[![Latest Version](https://img.shields.io/crates/v/pnmixer-rs.svg)](https://crates.io/crates/pnmixer-rs)
 [![Build Status](https://travis-ci.org/hasufell/pnmixer-rust.svg)](https://travis-ci.org/hasufell/pnmixer-rust)
 [![Join the chat at https://gitter.im/hasufell/pnmixer-rust](https://badges.gitter.im/hasufell/pnmixer-rust.svg)](https://gitter.im/hasufell/pnmixer-rust?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Documentation (master)](https://img.shields.io/badge/documentation-master-yellow.svg)](https://hasufell.github.io/pnmixer-rust/pnmixerlib/)