Update CONTRIBUTING.md with links, add content to README.md, and commit initial State Machine and Activity files

[jormello]

• Update CONTRIBUTING.md with links and add basic content to README.md.
• Commit initial files for State Machine and Activity resources.

The following repositories were referenced when determining which files to include:

• https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-networkfirewall
• https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-sso
• https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-ssm

Testing Pulled a fresh copy of the GitHub code, successfully ran mvn test, cfn generate, and mvn package for both Activity and State Machine resources. Ran cfn submit to locally deploy the resource handlers, then tested resource creation.

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