adding MariaDB to README.md (#10717)

MariaDB is mentioned as supported in requirements, so adding to README too. Also raises a discussion earlier:  #10686

Signed-off-by: Robert Silén <robert.silen@iki.fi>

README.md

@@ -19,7 +19,7 @@ What is this?
 K3s is a [fully conformant](https://github.com/cncf/k8s-conformance/pulls?q=is%3Apr+k3s) production-ready Kubernetes distribution with the following changes:
 
 1. It is packaged as a single binary.
-1. It adds support for sqlite3 as the default storage backend. Etcd3, MySQL, and Postgres are also supported.
+1. It adds support for sqlite3 as the default storage backend. Etcd3, MariaDB, MySQL, and Postgres are also supported.
 1. It wraps Kubernetes and other components in a single, simple launcher.
 1. It is secure by default with reasonable defaults for lightweight environments.
 1. It has minimal to no OS dependencies (just a sane kernel and cgroup mounts needed).