Add usage badge and update README heading

Added a badge for projects using Infinispan and updated the heading.

Author: karesti

README.md

@@ -1,7 +1,9 @@
-# Infinispan Simple Tutorials
+[![Used by](https://img.shields.io/badge/used%20by-Keycloak%20|%20Hibernate%20|%20Debezium%20%26%20more-003D6B?style=for-the-badge&logo=opensourceinitiative&logoColor=white)](https://github.com/infinispan/infinispan/network/dependents)
 
 ⭐ If you find these tutorials helpful, please star this repo, it helps others discover Infinispan!
 
+# Infinispan Simple Tutorials
+
 This is a collection of simple tutorials that explain how to use certain
 features of Infinispan in the most straightforward way possible.