Clarify directive filetypes

Small clarification about directive filetypes. The "valid" part was just adding confusion.

#### Allowing the usage of directives (includes) in additional file types
If you want to use directives (includes) in your own custom file extensions you can just add them to the array `lingon.config.directiveFileTypes`.
By default the following file types are registered: `['.js', '.less', '.css', '.ejs', '.html', '.md']`

```js
#!/usr/bin/env node
var lingon = require('lingon');

lingon.config.directiveFileTypes.push('.ngt', '.coffee');
```