Add docs for volumes_from

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Author: bfirsh

docs/yml.md

@@ -54,6 +54,11 @@ expose:
 volumes:
  - cache/:/tmp/cache
 
+-- Mount all of the volumes from another service or container
+volumes_from:
+ - service_name
+ - container_name
+
 -- Add environment variables.
 -- Environment variables with only a key are resolved to values on the host 
 -- machine, which can be helpful for secret or host-specific values.