chore(contributing): link to cli-integ package readme for integ tests instructions (#455)

The current instructions are out-of-date, lets keep one source of truth.

---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Author: iliapolo

CONTRIBUTING.md

@@ -47,20 +47,7 @@ This will build and unit test all packages.
 
 ## Integration tests
 
-Integration tests for this package are in a separate repository. They can be found here:
-
-<https://github.com/aws/aws-cdk-cli-testing>
-
-To make a change that involves the integration tests, make sure you have this package checked out.
-
-Run the tests locally by doing the following:
-
-```shell
-$ cd /path/to/aws-cdk-cli-testing/packages/@aws-cdk-testing/cli-integ
-$ bin/run-suite -s ../../../../aws-cdk-cli cli-integ-tests -t 'TEST NAME'
-```
-
-`-s` points to the root of the `aws-cdk-cli` repository.
+See [CDK CLI integration test](./packages/@aws-cdk-testing/cli-integ/README.md)
 
 ### Submitting a PR that involves changes to integration tests
 
@@ -76,4 +63,4 @@ the two repositories, and follow the following process:
 5. Release the testing PR.
 
 We will build automation to make this process smoother as soon as possible, but
-this is what it is for now.
+this is what it is for now.