Documentation to mention additonal config if using gzip plugin

Love the plugin and the rest of ember-cli-deploy - thank you.

I just ran into an issue where my `index.json` file was gzipped along with other resources which obviously caused problems.

I don't believe this to be a bug but maybe worth adding to the docs / readme that if using in conjunction with ember-cli-deploy-gzip that you should add the following to your `deploy.js` file:

```
gzip: {
ignorePattern: 'index.json'
}
```