Add "swift" to top-level README
Add "swift" to top-level README

Swift was missing from the list of preview SDKs. Now it's not.
awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples,awsdocs/aws-doc-sdk-examples