doc(installing-workflow): no periods in S3 bucket names

Warn users not to use periods in S3 bucket names in installing-workflow/configuring-object-storage.md. Related to issue deis#700

src/installing-workflow/configuring-object-storage.md

@@ -17,7 +17,7 @@ The helm chart for Deis Workflow can be easily configured to connect Workflow co
 
 Create storage buckets for each of the Workflow subsystems: `builder`, `registry`, and `database`.
 
-Depending on your chosen object storage you may need to provide globally unique bucket names.
+Depending on your chosen object storage you may need to provide globally unique bucket names. If you are using S3, do not use periods in the `registry_bucket` or `database_bucket` names; for example, `registry_bucket: "your.registry.bucket.name"` will fail, but `registry_bucket: "your-registry-bucket-name"` will work.
 
 If you provide credentials with sufficient access to the underlying storage, Workflow components will create the buckets if they do not exist.