Add document for Department

Fix: #1823

docs/modules/ROOT/nav.adoc

@@ -1,6 +1,7 @@
 * xref:index.adoc[Home]
 * xref:installation.adoc[Installation]
 * xref:usage.adoc[Usage]
+* xref:departments.adoc[Departments]
 * xref:cops.adoc[Cops]
 * xref:upgrade_to_version_2.adoc[Upgrade to 2.x]
 * xref:third_party_rspec_syntax_extensions.adoc[RSpec syntax extensions in third-party gems]

docs/modules/ROOT/pages/departments.adoc

@@ -0,0 +1,80 @@
+= Departments
+
+RuboCop RSpec currently has four departments:
+
+* xref:cops_rspec.adoc[RSpec]: This department checks for RSpec-specific code style.
+* xref:cops_rspec_capybara.adoc[RSpec/Capybara]: This department checks for Capybara-specific code style.
+* xref:cops_rspec_factorybot.adoc[RSpec/FactoryBot]: This department checks for factory_bot-specific code style.
+* xref:cops_rspec_rails.adoc[RSpec/Rails]: This department checks for RSpec-specific code style in Rails applications.
+
+Each department has its own set of cops. You can enable or disable individual cops within each department. If your project doesn't use factory_bot, and uses Sinatra and not Rails, you can disable the RSpec/FactoryBot and RSpec/Rails departments in your `.rubocop.yml` file:
+
+[source,yaml]
+----
+RSpec/FactoryBot:
+  Enabled: false
+
+RSpec/Rails:
+  Enabled: false
+----
+
+== Planned to extract to separate gems
+
+There are plans for extracting the following three departments before RuboCop RSpec v3.0:
+
+[cols="1, 1, 1"]
+|===
+|Department |Extract to |Status
+
+|RSpec/Capybara
+|https://github.com/rubocop/rubocop-capybara[rubocop-capybara]
+|Extracted in https://github.com/rubocop/rubocop-rspec/releases/tag/v2.18.0[v2.18.0]
+
+|RSpec/FactoryBot
+|https://github.com/rubocop/rubocop-factory_bot[rubocop-factory_bot]
+|Extracted in https://github.com/rubocop/rubocop-rspec/releases/tag/v2.22.0[v2.22.0]
+
+|RSpec/Rails
+|rubocop-rspec-rails
+|Not yet extracted
+|===
+
+=== Migration manual
+
+If you are using the RSpec/Capybara, RSpec/FactoryBot, or RSpec/Rails departments, you need to install the corresponding gem and add it to your `.rubocop.yml` file.
+
+For example, if you are using the RSpec/Capybara department, you need to install the `rubocop-capybara` gem and add it to your `.rubocop.yml` file:
+
+[source,ruby]
+----
+# Gemfile
+group :test do
+  gem 'rubocop-rspec'
+  gem 'rubocop-capybara'
+end
+----
+
+[source,yaml]
+----
+Capybara:
+  Enabled: true
+----
+
+And you need to disable the old department in your `.rubocop.yml` file:
+
+[source,yaml]
+----
+RSpec/Capybara:
+  Enabled: false
+----
+
+For another example, if you are not using these departments, you don't need to do anything.
+And when you update to RuboCop RSpec v3.0.0, you need to remove the old departments from your `.rubocop.yml` file, e.g.:
+
+[source,yaml]
+----
+RSpec/Capybara:
+  Enabled: false
+RSpec/FactoryBot:
+  Enabled: false
+----