demo - enhancements

[spicy-sauce]

1. Name the demo as dy-demo and all the containers with dy- prefix to avoid collisions:
   

ERROR: for postgres Cannot create container for service postgres: Conflict. The container name "/postgres" is already in use by container "c9f4d1600c4c2c569c5196b216817f2399890dc4a56015efd2f069c356a7ca48". You have to remove (or rename) that container to be able to reuse that name.
ERROR: Encountered errors while bringing up the project.

2. Change the network name from service to something more meaningful, dy-demo or datayoga or something.

3. Add a section named Demo in the main READMD.md file and point to the README.md under demo with a short explanation.