Move gem(puma) to group :production with instructions

[nimmolo]

Because otherwise, Rails will try to run puma in dev.

After this is merged, each person has to run this command locally (this is also a note in the Gemfile now):
bundle config set --local without 'production'

then bundle.

Then you can run rails s under webrick without further hassles.

[coveralls]

coverage: 94.41%. remained the same
when pulling 76e07ca on puma-in-group-production
into 614cc4f on main.