Add bandit security badge to README

Author: timlnx

README.rst

@@ -7,6 +7,10 @@
 .. image:: https://github.com/timlnx/bitmath/actions/workflows/python.yml/badge.svg
    :target: https://github.com/timlnx/bitmath/actions/workflows/python.yml
 
+.. image:: https://img.shields.io/badge/security-bandit-yellow.svg
+   :target: https://github.com/PyCQA/bandit
+   :alt: Security Status
+
 .. image:: https://img.shields.io/github/issues/timlnx/bitmath?style=flat-square
    :target: https://github.com/timlnx/bitmath/issues
    :alt: Open issues