chore(tools): update README for spec2cdk with an example (#31935)

Without an example, it feels very difficult to use spec2cdk as you need a `--service` in order for the tool to pick up the temporary-schemas.

### Issue # (if applicable)

Not applicable

### Reason for this change

Just a usability clarification.

### Description of changes

I've updated only the README

### Description of how you validated changes

I read the changes in preview for markdown.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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

Author: iph

tools/@aws-cdk/spec2cdk/README.md

@@ -29,7 +29,9 @@ At a code level, import `@aws-cdk/spec2cdk/lib/cfn2ts` for a drop-in replacement
 ## Temporary Schemas
 
 You can import additional, temporary CloudFormation Registry Schemas to test new functionality that is not yet published in `@aws-cdk/aws-service-spec`.
-To do this, drop the schema file into `temporary-schemas/us-east-1` and it will be imported on top of the default model.
+To do this, drop the schema file into `temporary-schemas/us-east-1` ([e.g](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/resource-type-schemas.html)). After you add the schema file, run spec2cdk with the specified module and short name.
+As an example, if you were updating [AWS::KMS::Key](https://github.com/aws-cloudformation/aws-cloudformation-resource-providers-kms), you must run:
+`bin/spec2cdk --service AWS::KMS`.
 
 ## CLI