README: Update instructions

Hello there!

By default, if you edit 'Program' field in 'Edit tool' dialog, IDEA automatically fills 'Working directory' field with path of executable file. So, when you run CSScomb tool from context menu/menu toolbar/etc it doesn't pick neither local .csscomb.json file nor global (if you keep them in Home dir). The trick is set 'Working directory' to '$FileDir$'. 
Tested in RubyMine 6.xx, WebStorm 8.xx
Cheers.