docs: add full schema link (#1415)

Co-authored-by: Jan Čurn <[email protected]>

Author: TC-MO

apify-api/openapi/openapi.yaml

@@ -10,7 +10,9 @@ info:
     The Apify API (version 2) provides programmatic access to the [Apify
     platform](https://docs.apify.com). The API is organized
     around [RESTful](https://en.wikipedia.org/wiki/Representational_state_transfer)
-    HTTP endpoints.
+    HTTP endpoints. 
+    
+    You can download the complete OpenAPI schema of Apify API in the [YAML](http://docs.apify.com/api/openapi.yaml) or [JSON](http://docs.apify.com/api/openapi.json) formats. The source code is also available on [GitHub](https://github.com/apify/apify-docs/tree/master/apify-api/openapi).
 
     All requests and responses (including errors) are encoded in
     [JSON](http://www.json.org/) format with UTF-8 encoding,