README.md - clarify use of localhost gem [ci skip]

[MSP-Greg]

Description

The current README.md describes how to use the localhost gem in development, but implies that one can require it in a rackup file. That is incorrect, as it needs to be required in a configuration file.

Updated README.md to clarify.

Closes #2818
Closes #3068

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[MSP-Greg]

@dentarg

Thanks. I updated the 'sample' configuration file, now it's simply the following:

require 'localhost'
# configuration methods (from Puma::DSL) as needed

Do you think that's descriptive enough?

[dentarg]

Yes