update README with validation information

Author: JaredCE

README.md

@@ -71,6 +71,10 @@ Options:
 - [CORS](#cors)
 - [OWASP Secure Headers](#owasp)
 
+#### Validation
+
+- [Validation](#validator)
+
 ### OpenAPI Mapping
 
 | OpenAPI field                                             | Serverless field                                                                                                                                      |
@@ -934,6 +938,12 @@ methodResponse:
 
 This will set the `Cache-Control` Response Header to have a value of "no-store" rather than any value the OWASP Secure Headers Project recommends.
 
+## Validator
+
+Validation for the OpenAPI Description is now (as of 0.0.90) done by [Redocly](https://redocly.com/). This is a slightly less opinionated validator for an OpenAPI Description, it should result in less errors around "YAML Anchors". It's also a maintained library, and has support for OpenAPI 3.1.0 which I hope to be able to support very soon.
+
+I am making use of https://www.npmjs.com/package/@redocly/openapi-core, which I have been warned is likely to change. If you notice anything going wrong with validation of your OpenAPI Description, feel free to open an issue here. I make active use of this library, so will hopefully come across those issues too.
+
 ## Example configuration
 
 Please view the example [serverless.yml](test/serverless-tests/best/serverless.yml).