Add docs to use nunjucks-slim in browser

By default, nunjucksify requires nunjucks in browser, regular nunjucks.js is huge and [may be replaced with a slim version](http://mozilla.github.io/nunjucks/api.html#setup-2-always-precompile), because templates are already precompiled. To do so, one may set the following `browser` option in package.json:

```
"browser": {
    "nunjucks": "nunjucks/browser/nunjucks-slim"
}
```

I tried this and it worked for me.