add slack badge
[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1173115.svg?style=flat-square)](https://doi.org/10.5281/zenodo.1173115)
[![GitHub stars](https://img.shields.io/github/stars/nschloe/meshio.svg?style=flat-square&logo=github&label=Stars&logoColor=white)](https://github.com/nschloe/meshio)
[![PyPi downloads](https://img.shields.io/pypi/dm/meshio.svg?style=flat-square)](https://pypistats.org/packages/meshio)
+[![Slack](https://img.shields.io/static/v1?logo=slack&label=slack&message=chat&color=4a154b&style=flat-square)](https://app.slack.com/client/TTHR0U4JY/)
+
+https://img.shields.io/static/v1?logo=slack&label=slack&message=chat&color=4a154b&style=flat-square
There are various mesh formats available for representing unstructured meshes.
meshio can read and write all of the following and smoothly converts between them:
