Add NumFOCUS badge (#3719)

# Description

To complete our NumFOCUS onboarding, we must add their badge to our
repository and website. Adding the "Powered by NumFOCUS" badge to our
README.

# Changed Behaviour

N/A

# Fixes

N/A

## Type of change

Choose which options apply, and delete the ones which do not apply.

- Update to human readable text: Documentation/error messages/comments

README.rst

@@ -1,6 +1,6 @@
 Parsl - Parallel Scripting Library
 ==================================
-|licence| |docs| |NSF-1550588| |NSF-1550476| |NSF-1550562| |NSF-1550528| |CZI-EOSS|
+|licence| |docs| |NSF-1550588| |NSF-1550476| |NSF-1550562| |NSF-1550528| |NumFOCUS| |CZI-EOSS| 
 
 Parsl extends parallelism in Python beyond a single computer.
 
@@ -64,6 +64,9 @@ then explore the `parallel computing patterns <https://parsl.readthedocs.io/en/s
 .. |CZI-EOSS| image:: https://chanzuckerberg.github.io/open-science/badges/CZI-EOSS.svg
    :target: https://czi.co/EOSS
    :alt: CZI's Essential Open Source Software for Science
+.. |NumFOCUS| image:: https://img.shields.io/badge/powered%20by-NumFOCUS-orange.svg?style=flat&colorA=E1523D&colorB=007D8A
+    :target: https://numfocus.org
+    :alt: Powered by NumFOCUS
 
 
 Quickstart