Link to engine delegate setup instructions somewhere in readme

[lispingit]

I tried creating a segment integration project from scratch. While following the readme it says to put things in your engine delegate, but it wasn't immediately obvious how I was supposed to obtain the engine delegate code in the first place.

I'd suggest in "Requirements" section there could be an item for engine setup with a delegate, with a link to the engine instructions there.

In other places that refer to the delegate, you could link backward to the requirements, so

In your implementation of FactualEngineDelegate, simply add a few lines of code to the engineDidStartWithInstance method.

might read like

In your implementation of FactualEngineDelegate (see Requirements), simply add a few lines of code to the engineDidStartWithInstance method.

[alfredp]

Updated https://github.com/Factual/segment-analytics-factual-engine-ios#requirements