Documentation should mention puma dependency

[James D]

The setup instructions should probably mention running the gem install puma command, if the user doesn't already have it installed.

As someone unfamiliar with the distinction between puma and puma-dev, it took me a few hours to figure out that the error bash: line 23: exec: puma: not found was not because of a configuration error, but because puma-dev does not include puma. I initially assumed that puma-dev was mutually exclusive with puma because they were both described as servers! 😆

[Alan Norton]

#195 (merged) resolves this.