Update readme.md

Including instructions to allow for categories to be used in apps which integrate superdb.
Fixed code style.
Added the UIView+SDBScreenshot.m category to be able to generate a screenshot image from any view.
Readme: added a line to explain how the ".image" command works.
Merge branch 'develop' of https://github.com/Shopify/superdb into feature/image-display
Fixed an issue with the SuperDB Area WC: for some reason, we "cannot form weak reference to instance of class SuperDebugAreaWindowController" according to the runtime… Needs to be investigated more.