testsys-launcher(docs): update README to use CFN context

PR #831 switched from using parameterized roles for assuming the
testsys-admin role to assuming this role from context. Update
the README instructions for deploying to reflect this.

Signed-off-by: Gavin Inglis <[email protected]>

Author: ginglis13

deploy/testsys-launcher/README.md

@@ -13,8 +13,9 @@ cdk deploy
 ```
 
 The default role that can assume the `testsys-admin` role is "Administrator".
-During deployment, use `--parameter TestsysAssumerRole={MY-ROLE-NAME}` to specify
-a different role that can assume the `testsys-admin` role.
+To specify other roles to assume `testsys-admin`, modify `cdk.json` or use
+a comma separated set of roles via CDK context like
+`--context testsysAdminAssumedBy=<role1>,<role2>,<role3>`.
 
 To get the kubeconfig, assuming you are acting as the role that can assume `testsys-admin`,
 use the `aws eks update-kubeconfig` command with the `testsys-admin` role: