Document new RuboCop `inherit_mode` config option
Document new RuboCop `inherit_mode` config option

- These docs are somewhat irrelevant now as nothing is using
  govuk-lint, but they're a good central place to see what RuboCop
  config apps _should_ be using based on the `rubocop-govuk` gem.
- We found that for apps that used Ruby-only cops, the recent changes in
  gem version 3.1.0 to exclude `bin/` files from linting worked fine.
- For apps that used Rails cops as well, the changes didn't work for
  `bin/` but they did work for `db/schema.rb` which is excluded from
  linting in `config/rails.yml`.
- This is because RuboCop by default *overwrites* any previous
  definitions of a config option when it detects a later one:
  https://rubocop.readthedocs.io/en/latest/configuration/#merging-arrays-using-inherit_mode.
- Unfortunately, we'll have to specify this `inherit_mode` stanza in
  every app.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs