README: Recommend using Regexp filters for add_group

[splattael]

Using String filters can include wrong files to groups because it matches if the string is contained within the path.

For example:

  # This matches files like `./lib/foo.rb`
  # but also `./app/models/library.rb'
  add_group 'Libraries', 'lib'

  # Prefer Regexp filter instead:
  add_group 'Libraries', %r{^/lib/}

See also https://gitlab.com/gitlab-org/gitlab/-/issues/416910

Alternative

Instead, we could adjust the String filter to use start_with? instead of include? but that'd be a breaking change and also require users to prepend a leading slash to their configuration like:

  # Before
  add_group 'Libraries', 'lib'

  # After
  add_group 'Libraries', '/lib'

[splattael]

This is somewhat related to #1016.

👋 @amatsuda If you have, do you mind reviewing this MR? 🙏