Added --save-dev to readme
Added --save-dev to readme

I think it would make sense to add the `--save-dev` flag, since this would mean `standard` is installed when people clone and `npm install` a project. Since one advantage is that teams have a common base, this seems to be a typical use case.
SimenB/standard,JedWatson/happiness,LinusU/standard,standard/standard,LinusU/standard,feross/standard,SimenB/standard,feross/standard,JedWatson/happiness,standard/standard