LMW-96 architecture doc

@ngaskill Please, review this architecture documentation style. The diagrams can be shown in this branch: https://github.com/liferay/liferay-screens/tree/gh-pages/ios/Library/svg

@JorgeFerrer when you have some spare time, could you please follow the steps described in "Create your own screenlet" and check whether or not you're able to create a screenlet from scratch?

Thanks guys!