add yaml as json docs

docs/yaml_as_json.md

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+# YAML as JSON
+
+By setting the the `yaml_as_json` input option to `"true"`, you can treat YAML files as JSON so that they can be validated with the same schema as JSON files. This is useful as the [yaml validator](https://github.com/ketanSaxena/schema-validator) is rather limited in its capabilities. This means any matching YAML files will be validated with the configured `json_schema` rather than the `yaml_schema`.
+
+Here is an example of how to use this feature:
+
+```yaml
+- name: json-yaml-validate
+  id: json-yaml-validate
+  uses: GrantBirki/[email protected] # replace with the latest version
+  with:
+    yaml_as_json: "true" # enable yaml as json mode
+```