Add discord chat button.
Add discord chat button
<img src="http://nalgebra.org/img/logo_nalgebra.svg" alt="crates.io">
</p>
<p align="center">
+ <a href="https://discord.gg/vt9DJSW">
+ <img src="https://img.shields.io/discord/507548572338880513.svg?logo=discord&colorB=7289DA">
+ </a>
<a href="https://travis-ci.org/rustsim/nalgebra">
<img src="https://travis-ci.org/rustsim/nalgebra.svg?branch=master" alt="Build status">
</a>
