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.

Doesn't work:
```coffeescript
requireAppScript = (moduleName) -> require '../../' + moduleName

requireAppScript 'x'
```

Works:
```coffeescript
requireAppScript = (moduleName) -> require '../../' + moduleName

requireAppScript 'x'
require '../../x'
```