docs: add third bind format (#57)

Signed-off-by: mbshields <[email protected]>

Author: mbshields

docs/reference/stacker_file.md

@@ -179,13 +179,15 @@ Because of the odd behavior of `cmd` and `entrypoint`, and the inherited nature
 
 ### `binds`
 
-`binds`: specifies bind mounts from the host to the container. There are two formats:
+`binds`: specifies bind mounts from the host to the container. There are three formats:
 
     binds:
+        - /zomg
         - /foo/bar -> /bar/baz
-	      - /zomg
+        - source: /foo/bar
+          dest: /bar/baz
 
-The first format binds `/foo/bar` to `/bar/baz`, and the second binds host `/zomg` to container `/zomg`.
+The first format binds host `/zomg` to container `/zomg` while the second and third bind host `/foo/bar` to container `/bar/baz`.
 
 At this time there is no awareness of change for any of these bind mounts, so `--no-cache` should be used to re-build if the content of the bind mount has changed.