chore(docs): add front-end instructions to README

@nchapman r? did I miss anything?
Unrelated, but related... I'll throw this in the package.json next time I rebase that elusive PR.

```
"postinstall": "bower update --config.interactive=false -s",
```
&mdash; via https://github.com/mozilla/fxa-content-server/blob/master/package.json#L7