Add OpenSSF badge (#440)

Signed-off-by: F.N. Claessen <felix@seita.nl>

Readme.md

@@ -8,6 +8,7 @@
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Documentation Status](https://readthedocs.org/projects/flexmeasures/badge/?version=latest)](https://flexmeasures.readthedocs.io/en/latest/?badge=latest)
 [![Coverage](https://coveralls.io/repos/github/FlexMeasures/flexmeasures/badge.svg)](https://coveralls.io/github/FlexMeasures/flexmeasures)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/6095/badge)](https://bestpractices.coreinfrastructure.org/projects/6095)
 
 The *FlexMeasures Platform* is the intelligent & developer-friendly EMS (energy management system) to support real-time energy flexibility apps, rapidly and scalable.