Merge pull request #3230 from servo/cargoify

Cargoify servo
Update the README a bit.
Only pass extra link flags on OS X.

Fixes #3249
Merge pull request #3259 from metajack/update-readme

Update the README a bit.
Merge pull request #3260 from metajack/fix-cef-linux

Only pass extra link flags on OS X.
Update Arch Linux prerequisites for cargoify branch