Update distribution README.md with recommendations

Provide users with community recommendations around securing installations.

1. change admin password
2. Do not expose JMX or web to public internet
3. Disable protocols not needed
4. Use two-way SSL as best practice, etc.
5. Limit resources by namespace (aka policyEntry) and authorization entry

Start with the distribution README.md and review its existing installation and security guidance. Add community recommendations covering administrator password changes, limiting public exposure, disabling unused protocols, two-way SSL, and namespace or policyEntry resource and authorization limits. Done means the recommendations are clearly documented without changing implementation code.

Written by the indexing model from the issue text.