Add grammar for cfn-guard rulesets

[johnttompkins]

Issue #, if available:

Description of changes: ABNF definition of the cfn-guard ruleset grammar. Looked at the cfn-guard README and the parser to determine some of this. Still a work in progress so leaving as a draft for now to hone and collect feedback.

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[PatMyron]

Should environment variables be added?

[johnttompkins]

Should environment variables be added?

Good call. Will add that as a terminal case

[ammokhov]

this looks good if you are familiar with semantics but should we do a resource example?

[johnttompkins]

this looks good if you are familiar with semantics but should we do a resource example?

Resource example? like an example of a ruleset? the readme has several examples of valid rules: https://github.com/aws-cloudformation/cloudformation-guard/blob/master/README.md Do you think we should include more here? Was just shooting to scope this document to defining the grammar.

[ammokhov]

this looks good if you are familiar with semantics but should we do a resource example?

Resource example? like an example of a ruleset? the readme has several examples of valid rules: https://github.com/aws-cloudformation/cloudformation-guard/blob/master/README.md Do you think we should include more here? Was just shooting to scope this document to defining the grammar.

nice abnf addition, lgtm

syntax highlighted