Added note about require with more than string literals

Ran into this problem myself. It makes sense why it wouldn't work but took me awhile to realise it so it's worth noting.

Doesn't work:
```coffeescript
requireAppScript = (moduleName) -> require '../../' + moduleName

requireAppScript 'x'
```

Works:
```coffeescript
requireAppScript = (moduleName) -> require '../../' + moduleName

requireAppScript 'x'
require '../../x'
```