README: Clarify config file fallbacks [ci skip]

[olleolleolle]

Description

While reading the configuration file finding notes in the README, I was suddenly confused, and found the wording unclear.

This PR adds more words.

Implementation:

puma/lib/puma/control_cli.rb

Line 122 in d2f9d6d

@config_file = %W(config/puma/#{environment}.rb config/puma.rb).find do |f|

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