add a section about uploading files that don't exist.

website/source/docs/provisioners/file.html.md

@@ -47,9 +47,6 @@ The available configuration options are listed below. All elements are required.
 -   `direction` (string) - The direction of the file transfer. This defaults to
     "upload". If it is set to "download" then the file "source" in the machine
     will be downloaded locally to "destination"
-    
-The source file must exist before running Packer, this means that creating the 
-file with a provisioner does not work.
 
 ## Directory Uploads
 
@@ -78,6 +75,17 @@ directly.
 This behavior was adopted from the standard behavior of rsync. Note that under
 the covers, rsync may or may not be used.
 
+## Uploading files that don't exist before Packer starts
+
+In general, local files used as the source **must** exist before Packer is run.
+This is great for catching typos and ensuring that once a build is started,
+that it will succeed. However, this also means that you can't generate a file
+during your build and then upload it using the file provisioner later.
+A convenient workaround is to upload a directory instead of a file. The
+directory still must exist, but its contents don't. You can write your
+generated file to the directory during the Packer run, and have it be uploaded
+later.
+
 ## Symbolic link uploads
 
 The behavior when uploading symbolic links depends on the communicator. The