Update: Add CDK Toolkit API reference link

Author: evzzk

v2/toolkit-library.md

@@ -25,4 +25,6 @@ await cdk.deploy(cloudAssembly, {
 
 ## Get started with the CDK Toolkit Library<a name="toolkit-library-gs"></a>
 
-To get started, see the `[ReadMe](https://www.npmjs.com/package/@aws-cdk/toolkit-lib)` in the *@aws\-cdk/toolkit\-lib* `npm` package\.
+To get started, see the `[ReadMe](https://www.npmjs.com/package/@aws-cdk/toolkit-lib)` in the *@aws\-cdk/toolkit\-lib* `npm` package\.
+
+For API reference information, see the [CDK Toolkit Library API reference](https://docs.aws.amazon.com/cdk/api/toolkit-lib/)\.