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** Redis connection other then the default *redis://127.0.0.1:6379* for which I had to defined my Redis connection for Resque in initializer file like this 

```ruby 
  #config/initializer/resque_connection.rb
  Resque.redis = Redis.new( )
```
Now this **work well** with Resque and the Redis connection is picked up correctly but when **resque_scheduler**  rake task is invoked the my Redis connection is not taken into consideration and the rake tasks  dies/fail saying "Connection Timeout" which is correct  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 

```ruby 
rake environment resque_scheduler
```

And I don't see this documented any where hence doing the same