add initial content to README

Author: drew-russell

README.md

@@ -1,2 +1,19 @@
-# api-documentation
-API documentation for the Rubrik Cloud Data Management (CDM) platform
+# Rubrik REST API Documentation
+
+The Rubrik REST API provides a RESTful interface for working with Rubrik clusters and Rubrik Edge virtual appliances. The API can be used to query, configure, and control nearly all of the operations of the Rubrik software.
+
+If you have access to a Rubrik cluster, you may also utilize the Rubrik API Playground at:
+
+`https://{{node_ip}}/docs/{{ v1|internal }}/playground`
+
+_Note: `internal` API endpoints may change between CDM release versions._
+
+
+## CDM v4.2.1
+
+* [v1](https://172.21.8.53/docs/v1/)
+* [internal](https://github.com/rubrikinc/api-doc-internal)
+
+
+
+