docs: add OpenSSF Best Practices registration badge

Author: hyperpolymath

README.adoc

@@ -1,4 +1,8 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 = k9-pre-commit
+image:https://img.shields.io/badge/OpenSSF-Best_Practices-green?logo=openssourcesecurity[OpenSSF Best Practices,link="https://www.bestpractices.dev/en/projects/new?repo_url=https://github.com/hyperpolymath/k9-pre-commit"]
+
 :toc: preamble
 :icons: font