added the extra_hosts option to the yml configuration which exposes the --add-host flag from the docker client

Signed-off-by: Sam Wing <sampwing@gmail.com>

docs/yml.md

@@ -87,6 +87,23 @@ external_links:
  - project_db_1:postgresql
 ```
 
+### extra_hosts
+
+Add hostname mappings. Use the same values as the docker client `--add-hosts` parameter.
+
+```
+extra_hosts:
+ - docker: 162.242.195.82
+ - fig: 50.31.209.229
+```
+
+An entry with the ip address and hostname will be created in `/etc/hosts` inside containers for this service, e.g:
+
+```
+162.242.195.82  docker
+50.31.209.229   fig
+```
+
 ### ports
 
 Expose ports. Either specify both ports (`HOST:CONTAINER`), or just the container