Recommend bucket versioning for S3 remote state. (hashicorp#6456)

Author: clstokes

website/source/docs/state/remote/s3.html.md

@@ -14,6 +14,10 @@ Stores the state as a given key in a given bucket on [Amazon S3](https://aws.ama
 make them included in cleartext inside the persisted state.
 Use of environment variables or config file is recommended.
 
+~> **Warning!** It is highly recommended to enable
+[Bucket Versioning](http://docs.aws.amazon.com/AmazonS3/latest/UG/enable-bucket-versioning.html)
+on the S3 bucket to allow for state recovery in the case of accidental deletions and human error.
+
 ## Example Usage
 
 ```