Add usable instructions for running locally

Author: martypdx

README.md

@@ -19,8 +19,12 @@ $ npm install
 $ grunt
 ```
 
-Once you've built the site, open `build/index.html` in your web browser to preview.
+Running `grunt` will build the site and rebuild when docs changes. You need to use a local webserver to preview in your web browser. One easy option is to use [http-server](https://www.npmjs.org/package/http-server) (installable via `npm install -g http-server`). You need to run it from the `/build` directory and explicitly include `html` as the default extension (add `-s` for silent mode):
 
+```sh
+docs.ractive.org/build>http-server -e html
+```
+Open `localhost` on the port specified (usually `8080`), and it should redirect to `localhost:8080/latest/get-started`. Refresh to pick up changes made in docs.
 
 Contributing
 ------------