Merge pull request #3318 from aldesantis/chore/api-contributors-guide

Document contribution guidelines for API documentation

api/README.md

@@ -9,3 +9,17 @@ Run the API tests:
 ```bash
 bundle exec rspec
 ```
+
+## Documentation
+
+The API documentation is in the [openapi](https://github.com/solidusio/solidus/tree/master/api/openapi)
+directory. It follows the OpenAPI specification and it is hosted on
+[Stoplight](https://solidus.docs.stoplight.io/).
+
+If you want to contribute, you can use [Stoplight Studio](https://stoplight.io/p/studio),
+an OpenAPI editor, to edit the files visually, and copy-paste the 
+resulting code into the `openapi` directory.
+
+CircleCI automatically syncs our Git repo with Stoplight when a PR is
+merged, and automatically publishes a new version on Stoplight when
+a new Solidus version is released.