Update Readme

- Add responsive related details; viewport widths and orientations
- Add descriptive text to .navicon-extended
- Add photoshop mockup of the body of the site; WIP

To do:
-.navicon-extended text is not centered vertically; must fix
- add shape to .navicon-extended, to create "chat bubble"-like visual
- give .navicon* classes their own unique IDs; perhaps create .navicon
class that encompasses all of them