Add example of using EC2 instance tags for authorisation

[aidansteele]

Should include the authoriser lambda function, but more importantly example IAM policies needed to ensure that users can't escalate their own access. See:

• https://aws.amazon.com/blogs/aws/new-tag-ec2-instances-ebs-volumes-on-creation/
• http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ExamplePolicies_EC2.html#iam-example-taggingresources
• http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-policy-structure.html#amazon-ec2-keys
• https://stackoverflow.com/questions/43121514/ec2-iam-policy-to-require-tags
• http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ec2-api-permissions.html#ec2-api-unsupported-resource-permissions
• https://aws.amazon.com/premiumsupport/knowledge-center/iam-ec2-resource-tags/