Coinbase address not set error on startup

[tynes]

I get an error No addresses available for coinbase. if I follow the instructions in the readme if I do not set the flag --coinbase-address. I don't think that its immediately obvious for somebody new to the codebase that the --coinbase-address flag would fix it. Maybe that could be added to the readme?

It would also be ideal to generate a new coinbase address each for each successive block found, to prevent address reuse. That is definitely a nice to have.