add support for nginx

fix #74
Good question. Personally, I like a config reference in bundle docs, so I can quickly see all the config options a bundle offers and find the one I’m looking for. On the other hand, the relevant config should be contained with each feature to get a more story-like reading: configure this, then get that. 

So I propose we put all config options into a separate reference file, and then link there or include the relevant part in each feature section.