Add logo (#81)

Author: manzt

README.md

@@ -1,9 +1,12 @@
-# vizarr
+<p align="center">
+  <img width="400" src="./assets/logo-wide.png" alt="Vizarr">
+</p>
+
 [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/hms-dbmi/vizarr/master?filepath=example%2Fgetting_started.ipynb)
 [![launch ImJoy](https://imjoy.io/static/badge/launch-imjoy-badge.svg)](https://imjoy.io/#/app?workspace=vizarr&plugin=https://github.com/hms-dbmi/vizarr/blob/master/example/VizarrDemo.imjoy.html)
 [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/hms-dbmi/vizarr/blob/master/example/mandelbrot.ipynb)
 
-![Multiscale OME-Zarr in Jupyter Notebook with Vizarr](/screenshot.png)
+![Multiscale OME-Zarr in Jupyter Notebook with Vizarr](./assets/screenshot.png)
 
 Vizarr is a minimal, purely client-side program for viewing Zarr-based images. It is built with 
 [Viv](https://github.com/hms-dbmi/viv) and exposes a Python API using the