docs: update readme (#855)

Author: kcadette

README.md

@@ -1,5 +1,7 @@
 # FHIR Works on AWS deployment
 
+# This GitHub repository has been migrated. You can now find FHIR Works on AWS at https://github.com/aws-solutions/fhir-works-on-aws.
+
 ## Upgrade notice
 
 Versions 3.1.1 and 3.1.2 of the `fhir-works-on-aws-authz-smart` package have been deprecated for necessary security updates. Please upgrade to version 3.1.3 or higher. For more information, see [the fhir-works-on-aws-authz-smart security advisory](https://github.com/awslabs/fhir-works-on-aws-authz-smart/security/advisories/GHSA-vv7x-7w4m-q72f).