update README with local testing instructions

Author: whummer

README.md

@@ -89,4 +89,18 @@ eb deploy
 eb open
 ```
 
+## Local Testing
 
+This section contains instructions on how to test the application locally (using mocked services instead of the real AWS services).
+
+### LocalStack
+
+To test this application using [LocalStack](https://github.com/localstack/localstack), you can use the `awslocal` CLI (https://github.com/localstack/awscli-local).
+```
+pip install awscli-local
+```
+Simply parameterize the `./createResources.sh` installation script with `aws_cmd=awslocal`:
+```
+cd aws; aws_cmd=awslocal ./createResources.sh
+```
+Once the code is deployed to the local S3 server, the application is accessible via http://localhost:4572/cognitosample-localapp/index.html (Assuming "localapp" has been chosen as resource name in the previous step)