Add numfocus shield

Author: dpvc

README.md

@@ -1,5 +1,6 @@
 # MathJax (Source Repository)
 <img class="shield" alt="GitHub release version" src="https://img.shields.io/github/v/release/mathjax/MathJax-src?sort=semver"> <img class="shield" alt="GitHub release version (branch)" src="https://img.shields.io/github/package-json/v/mathjax/MathJax/legacy-v2?label=release v2">
+<a class="reference external image-reference" href="http://www.numfocus.org"><img alt="powered-by NumFocus" src="https://img.shields.io/badge/powered%20by-NumFOCUS-orange.svg?style=flat&amp;amp;colorA=E1523D&amp;amp;colorB=007D8A"></a>
 
 ## Beautiful math in all browsers