add README instructions for test credentials required for presigned U…

…RLs (localstack#3198)

README.md

@@ -303,6 +303,12 @@ aws configure --profile default
 
 # Config & credential file will be created under ~/.aws folder
 ```
+**NOTE**: Please use `test` as Access key id and secret Access Key to make S3 presign url work. We have added presign url signature verification algorithm to validate the presign url and its expiration. You can configure credentials into the system environment using `export` command in the linux/Mac system. You also can add credentials in `~/.aws/credentials` file directly.
+
+```
+export AWS_ACCESS_KEY_ID=test
+export AWS_SECRET_ACCESS_KEY=test
+```
 
 **NEW**: Check out [awslocal](https://github.com/localstack/awscli-local), a thin CLI wrapper
 that runs commands directly against LocalStack (no need to specify `--endpoint-url` anymore).