Add contributing.md to the repository and update the readme accordingly.
Update contributing.md

Minor tweak on list formatting so it is picked up by parsers.
Initial commit of device mode docs.
Removed of mobile-emulation page and files. Updating _preview.html and tips-and-tricks.html to reference device-mode in favor of mobile-emulation
Mobile emulation links switched to device mode
correct Save All Files shortcut. fixes crbug.com/401998
Merge pull request #124 from Garbee/contributing

Add contributing.md to the repository and update the readme accordingly.
Merge pull request #116 from Garbee/new-emulation

New emulation docs
add ' & Mobile Emulation' to Device Mode headers/links
updates to device mode docs. new images. more complete port of old docs.
Merge pull request #128 from GoogleChrome/devicemode-paul

Device Mode doc update
Merge pull request #127 from Garbee/remove-mobile-emulation

Removed of mobile-emulation page and files.