Adapter docs for convention-breakers

[TheDeveloper]

Thought this could be useful when using Step with libraries that don't observe the callback(err,res) convention.

Wdyt?

[creationix]

Good idea, but using the inline self-calling function is terribly confusing.

[TheDeveloper]

True - I've clarified it a bit. Untested but should work.

[creationix]

Better would be to simply convert the function to be node-style without involving step at all. I don't like using bind to insert step's "this" into the wrapper.