Added security considerations document

SECURITY_CONSIDERATIONS.md

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+# Security Considerations
+
+OpenAPI documents use JSON, YAML and JSON Schema and therefore share their same security considerations. [JSON Schema Core](https://json-schema.org/draft/2020-12/json-schema-core#section-13) [JSON Schema Validation](https://json-schema.org/draft/2020-12/json-schema-validation#name-security-considerations) [YAML](https://www.ietf.org/archive/id/draft-ietf-httpapi-yaml-mediatypes-10.html) [JSON](https://www.rfc-editor.org/rfc/rfc8259)
+In additional, OpenAPI documents are processed by a wide variety of tooling for numerous different purposes, such as client code generation, documentation generation, service side routing, and API testing. OpenAPI document authors must consider the risks of the scenarios where the OpenAPI document may be used.
+OpenAPIs document may contain references to external resources that may be dereferenced automatically by consuming tools. References in an OpenAPI document, or across OpenAPI documents may cause a cycle. Tooling must detect and handle cycles to prevent resource exhaustion.
+Certain properties allow the use of Markdown which can contain HTML including script. It is the responsibility of tooling to appropriately sanitize the Markdown.