Update documentation

[bdshie]

Is your feature request related to a problem? Please describe.
cfn-nag is a linting tool that is not owned by AWS or CloudFormation team and has been a stale project for a while. It is still

Describe the solution you'd like
I would like suggest to make these mandatory instead:

• cfn-lint for syntax: https://github.com/aws-cloudformation/cfn-lint
• cfn-guard: https://github.com/aws-cloudformation/cloudformation-guard -- there is cfn-nag equivalent rules stored in Guard Rules Registry: https://github.com/aws-cloudformation/aws-guard-rules-registry