Document 'auto_start' in fig.yml

Signed-off-by: Chris Corbyn <chris@w3style.co.uk>

docs/yml.md

@@ -54,6 +54,9 @@ expose:
 volumes:
  - cache/:/tmp/cache
 
+-- Do not automatically run this service on `fig up` (default: true)
+auto_start: false
+
 -- Add environment variables.
 environment:
   RACK_ENV: development