fix(rds): remove Instance class (#2081)

There was an instance class which was a placeholder, and didn't have any
implementation. Remove to avoid confusing users.

Also update the README to be very explicit that RDS is not
well-supported.

packages/@aws-cdk/aws-rds/README.md

@@ -2,6 +2,13 @@
 
 The `aws-cdk-rds` package contains Constructs for setting up RDS instances.
 
+> Note: the functionality this package is currently limited, as the CDK team is
+> focusing on other use cases first. If your use case is not listed below, you
+> will have to use achieve it using CloudFormation resources.
+>
+> If you would like to help improve the state of this library, Pull Requests are
+> welcome.
+
 Supported:
 
 * Clustered databases

packages/@aws-cdk/aws-rds/lib/index.ts

@@ -1,6 +1,5 @@
 export * from './cluster';
 export * from './cluster-ref';
-export * from './instance';
 export * from './props';
 export * from './cluster-parameter-group';
 export * from './rotation-single-user';