Adding documentation for how to use templates in content files

It seems obvious now, but I had to read [the source](https://github.com/segmentio/metalsmith-templates/blob/552c00f38975b8eb0cd7a49f9d2dc121fb905dbb/lib/index.js#L50) to discover that the `template` property is what controls the template for a given content file. I thought it would be helpful detail to include in the readme.