Add declarative OpenAPI example to curated section

Add Swift Package Registry OAS to curated integrations section.

Author: mattpolzin

README.md

@@ -23,6 +23,7 @@ A library containing Swift types that encode to- and decode from [OpenAPI](https
     - [Specification Extensions](#specification-extensions)
   - [Dereferencing & Resolving](#dereferencing--resolving)
 - [Curated Integrations](#curated-integrations)
+  - [Declarative OpenAPI Documents](#declarative-openapi-documents)
   - [Generating OpenAPI Documents](#generating-openapi-documents)
   - [Semantic Diffing of OpenAPI Documents](#semantic-diffing-of-openapi-documents)
 - [Notes](#notes)
@@ -225,6 +226,10 @@ Following is a short list of integrations that might be immediately useful or ju
 
 If you have a library you would like to propose for this section, please create a pull request and explain a bit about your project.
 
+### Declarative OpenAPI Documents
+
+[**Swift Package Registry API Docs**](https://github.com/mattt/swift-package-registry-oas) define the OpenAPI documentation for the Swift Package Registry standard using declarative Swift code and OpenAPIKit. This project also provides a useful example of producing a user-friendly ReDoc web interface to the OpenAPI documentation after encoding it as YAML.
+
 ### Generating OpenAPI Documents
 
 [**VaporOpenAPI**](https://github.com/mattpolzin/VaporOpenAPI) / [VaporOpenAPIExample](https://github.com/mattpolzin/VaporOpenAPIExample) provide an example of generating OpenAPI from a Vapor application's routes.