Update README with configuration instructions
Update README with configuration instructions

Changelog:
Introduces ability to configure Herodotus. Here's an example on how to
configure it:

    Herodotus::Configuration.run do |config|
      config.base_path          = '/path/to/project/root'
      config.changelog_filename = 'CHANGELOG.md'
    end

hgmnz/herodotus