Update html.md
Update html.md

Adds some docs about the web app manifest and modernizr 3.
Also, housekeeping!
roblarsen/html5-boilerplate,h5bp/html5-boilerplate,h5bp/html5-boilerplate,roblarsen/html5-boilerplate,rprouse/html5-boilerplate,JohnnyWalkerDesign/html5-boilerplate,Upmacis-Andrejs/12guru_Front-end,rprouse/html5-boilerplate,byog/html5-boilerplate,Upmacis-Andrejs/12guru_Front-end,JohnnyWalkerDesign/html5-boilerplate,byog/html5-boilerplate