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'
}
```

Start with the repository README or documentation referenced by the issue and check how ember-cli-deploy-gzip configuration is described. Document that users should add the shown ignorePattern for index.json in deploy.js, then review the rendered wording for clarity and completeness.

Written by the indexing model from the issue text.