There are three parts changed:
1. Add producer_consumer.md and update the README.md in root dir.
2. Add producer_consumer package:
We can use assign tasks to producer after producer starts.
Producer has limit buffer for task. If the buffer is full and no
consumer to take, the producer will ignore the task.
Consumer needs to regiester if it wants to do task for producer.
All the communication is based on RPC.
3. Add example for producer_consumer package.
The example will show the case that the buffer of producer is full.
