Gutenberg Mobile's readme had some copy in the troubleshooting section that wasn't included in Gutenberg's how-to: https://github.com/wordpress-mobile/gutenberg-mobile/edit/trunk/README.md

This commit fixes that issue by updating Gutenberg's how-to with the latest from Gutenberg Mobile.

With this commit, I've tidied up some of the copy, attempting to make the instructions as concise and helpful as possible. I've also set the images to the same width to tidy up the overall look of the document (the images will appear small on mobile devices, but can be tapped on to get the full image).

- The "Unit Tests" is moved towards the bottom of the document, so that's it's closer to the instructions for integration tests. the document flows as follows:
- The steps for installing Xcode are moved under the "iOS" section.
- The overall flow of the document has been updated to the following: Clone Gutenberg > iOS > Android > Tests

Some versions of Ruby may not require the `ffi` to be manually installed, which is noted with this commit.