Standalone apps guidance

This is a first pass at converting Tom's presentation into more coherent written guidance. It still needs work and I'm not 100% happy with the terms or definitions contained herein (I will add line-specific comments), although it is aimed at a non-technical audience, so it shouldn't go into too much pedantic detail.

I may add some illustrations to the guidance if thought necessary (examples of web apps vs native apps, etc)

There will also be a smart answer-esque questionnaire to sit alongside this that service managers can use to test if they have a case for creating a native app.