Add some more docs on `include_relative`

Well, more docs, and one more test, to ensure that `include_relative ../<path>` really truly does not work.
why not

```ruby
"{% #{tag_name} file.ext param='value' param2='value' %}"
```

? Just have to ensure `tag_name` (or `@tag_name`) is set properly in `initialize` and you're goooood to go.