Update hack guide for UI

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

contrib/HACK.md

+## Build a development API Gateway
+
+Create `functions/gateway:latest-dev`
+
+```
+$ cd gateway/
+$ ./build.sh
+```
+
+Now edit the gateway service in your `docker-compose.yml` file and deploy the stack.
+
+If you want to use an overriden name then pass in the tag to the `./build.sh` script such as `./build.sh test-1`.
+
 ## Hack on the UI for the API Gateway
 
 To hack on the UI without rebuilding the gateway mount the assets in a bind-mount like this:
 
+Remove the Docker stack, then create the faas network as "attachable":
+
 ```
+$ docker stack rm func
 $ docker network create func_functions --driver=overlay --attachable=true
+```
+
+Now edit the `docker-compose.yml` file and add an attribute to the `functions` network of `attachable`.
+
+Now you can run the gateway as its own container and bind-mount in the HTML assets.
+
+```
+$ cd faas
 $ docker run -v `pwd`/gateway/assets:/root/assets -v "/var/run/docker.sock:/var/run/docker.sock" \
--p 8080:8080 --network=func_functions -ti alexellis2/faas-gateway:latest-dev
+-p 8080:8080 --network=func_functions -ti functions/gateway:latest-dev
 ```
 
-Then edit `docker-compose.yml` to use an external network and do a `./deploy_stack.sh`.
+Now deploy the rest of the stack with:  `./deploy_stack.sh`.