fix docs so they refer to start subcommand instead of hello

[r-arias]

Hi,

I stumbled across the docs referring to the hello subcommand, when in the current version the default is the start subcommand. I went ahead and fixed the README.md.

I was unsure whether to change config.rs.hbs, as well, and now I am unsure about all of it. The start command actually does just say "hello", so maybe, instead the command should be renamed to hello again?

Either way, this PR makes the docs match the actual command that can be run, so it should hopefully be an improvement.

Thanks for building this!
Cheers,
R

PS: I've read the contributing guidelines. I don't think it'd be appropriate to be listed under authors for this trivial change. Let me know if it's absolutely vital. In any case, I license my "contribution" under the terms of the Apache License, Version 2.0.