Add docs and provide a reference implementation on RSpec DSL aliases configuration in third-party gems

[pirj]

With #956 being released as part of RuboCop RSpec 2.0, this opens a possibility for third-party gems to supplement the default RSpec aliases configuration with those aliases that they introduce.

The purpose of this is to push the responsibility of configuring RSpec alias detection to third-party gems to avoid potential conflicts, avoid an excessive default list of aliases, and reduce the maintenance burden on us.

TODO:

• provide a reference implementation in one of the repos
• add docs on how third-party gems can provide their users that also use RuboCop RSpec with alias configuration they introduce

Notable examples are:

• cancancan's is_able_to, see: RSpec/EmptyExampleGroup raises when using only shared examples in a context #884
• action_policy's describe_rule, failed, succeed, see: Make RSpec describe_rule DRY palkan/action_policy#103, pull: Add RuboCop RSpec support palkan/action_policy#138
• test-prof's let_it_be/before_all, pull: Add RuboCop RSpec support test-prof/test-prof#199
• rspec-its's its
• saharspec's its_map, its_block, its_call, see: RSpec/SharedContext: needs CustomIncludeMethods setting #975
• pundit's permissions (also role was mentioned), see: Add a spec for the project policy openSUSE/open-build-service#5878, Users should be able to globally configure example group and includes language #333
• rspec-rails 's setup hook Allow to catch RSpec/ExpectInHook offenses in the setup block #835 - seemingly setup comes from the underlying Minitest

Proposed solution

https://docs.rubocop.org/rubocop/1.2/configuration.html#inheriting-configuration-from-a-dependency-gem

Third party gem's rubocop_rspec_alias_config.yml:

RSpec:
  Language:
    Includes:
      Examples:
        - is_able_to

Third-party gem's README.md:

## Usage with RuboCop RSpec

Please add the following to your `.rubocop.yml`:
----
inherit_gem:
  third-party-gem: rubocop_rspec_alias_config.yml
----

And update your `gemspec` file to include `rubocop_rspec_alias_config.yml` in `spec.files`.

Source of infromation: #333