Updates to README.md Install instructions (#1389)

This PR updates and clarifies the condenser install instructions.

- Added step to verify NVM is installed.
- Changed config file from steem-example.json to default.json.
- Added quit/exit commands where appropriate.
- Added step to update ~/condenser/db/config/config.json with the mysql password.
- Clarified the step at the end to know when condenser is running.
- Updated the port number to be based on whatever is output as the port number condenser is running on.