modular options docs update

[Kai Tran]

It took me a while to realize the option modular: true is not part of the riot compiler but a custom option of this gulp plugin.

Within the plugin additional code gets prepended as

if (options.modular) {
      compiledCode = ...
}

Imo it should be more clear that the modular option isn't an option of the actual riot compiler. The docs shouldn't link to the riot docs when describing it or rather point out it's a custom option.

Would you mind to update the docs?

[jigsaw]

yes, pull requests welcome!