Add documentation about Observer's state management

[lalitpagaria]

Currently each Observer support state management.

Obsei store state in any of sqlalchemy supported database on schema mentioned in WorkflowTable class.

There is example also exist to show it's capability.

State management is particularly useful in using Obsei in worker environment. For example a cron fetch tweets from Twitter every 1 hour. With state management Obsei's Observer do not read duplicate tweets.

Let's add documentation about this into Obsei docs.