Add a slighly cheesy contributors panel to readme (#6520)

It's good so many people have contributed!

Author: max-sixty

README.md

@@ -99,6 +99,12 @@ released as open source in May 2014. The project was renamed from
 "xray" in January 2016. Xarray became a fiscally sponsored project of
 [NumFOCUS](https://numfocus.org) in August 2018.
 
+## Contributors
+
+Thanks to our many contributors!
+
+[![Contributors](https://contrib.rocks/image?repo=pydata/xarray)](https://github.com/pydata/xarray/graphs/contributors)
+
 ## License
 
 Copyright 2014-2019, xarray Developers