Recognize more CloudFormation templates

[PatMyron]

cfn-lint-atom/lib/atom-cfn-lint.js

Lines 144 to 151 in d7afcd2

	const isCfnRegex = new RegExp('"?AWSTemplateFormatVersion"?')
	let isCfn = false
	activeEditor.buffer.buffer.getLines().forEach((line) => {
	if (isCfnRegex.exec(line)) {
	isCfn = true
	}
	})

Update README too:

cfn-lint-atom/README.md

Lines 12 to 13 in 2f3c511

	#### JSON/YAML Templates
	Atom-cfn-lint will work with JSON and YAML files but the goal is to only scan CloudFormation templates. As a result we look for `AWSTemplateFormatVersion` being defined in the file. CloudFormation only requires `Resources` to be defined but this is too generic for assuming that the file is a CloudFormation template.

---

Similar discussions: aws-cloudformation/cfn-lint#1171, aws-cloudformation/cfn-lint-visual-studio-code#99, aws/aws-toolkit-jetbrains#1715 (comment), https://github.com/github/super-linter/pull/231#discussion_r445708695, mwpearce/vscode-cfn-nag#6, joenye/coc-cfn-lint#3 (comment)