added description in README doc for rake task to load with environment

Added description in README doc for rake task to load with environment

Well Personally I have do this because my `Resque` was using a different connection other then the default `redis://127.0.0.1:6379` so I set my Redis connection to Resque in initializer like this 

`Resque.redis = Redis.new( )` 

Now this work well with resque and redis connection is picked up correctly but when resque_scheduler without environment it does not take the redis connection into consideration and dies of "Connection Timeout" because `resque_scheduler` rake task does not load the environment so in use case like this it is important to load the rake task along with environment something like this 

`rake environment resque_scheduler`