Adds synchronized cross-device testing

For #527 - adds Synchronized Cross-device Testing article. Reviewed by @dr4b and @paulirish in GDocs, but please do feel free to let me know if anything at all needs to be changed.

Note: assets here haven't been run through imageoptim yet. I'm not sure if that's something we should do post-merge or whether there's a replacement optimization setup in place now that we're no longer using PS. Any advice here would be great.

Cheers!