Updated README

There was this error, which I fixed.
"Error: No available formula for browsertime"
Merge pull request #364 from antonbabenko/patch-1

Updated README - "Error: No available formula for browsertime" thanks @antonbabenko
cleanup
Merge branch 'master' of github.com:sitespeedio/sitespeed.io
upgraded to latest browsertime
2.5.6
made it clearer about the YSlow js file
added Tobias & made yslow contribution even clearer [skip ci]
even cleaner YSlow contrib guide [skip ci]
Merge pull request #1 from sitespeedio/master

Resync