Add Linux Foundation Health Score badge to README

Congrats! We have onboarded matplotlib to LFX Insights, the Linux Foundation's platform for monitoring the world's most critical open-source projects.

https://insights.linuxfoundation.org/project/matplotlib

Author: jonathimer

README.md

@@ -2,6 +2,7 @@
 [![Conda](https://img.shields.io/conda/vn/conda-forge/matplotlib)](https://anaconda.org/conda-forge/matplotlib)
 [![Downloads](https://img.shields.io/pypi/dm/matplotlib)](https://pypi.org/project/matplotlib)
 [![NUMFocus](https://img.shields.io/badge/powered%20by-NumFOCUS-orange.svg?style=flat&colorA=E1523D&colorB=007D8A)](https://numfocus.org)
+[![LFX Health Score](https://insights.linuxfoundation.org/api/badge/health-score?project=matplotlib)](https://insights.linuxfoundation.org/project/matplotlib)
 
 [![Discourse help forum](https://img.shields.io/badge/help_forum-discourse-blue.svg)](https://discourse.matplotlib.org)
 [![Gitter](https://badges.gitter.im/matplotlib/matplotlib.svg)](https://gitter.im/matplotlib/matplotlib)