AO3-6691 Add Readme for custom RuboCop cops (#4778)

* AO3-6691 Add Readme for custom RuboCop cops

* Fix typo

* Embed links in text

* Remove duplicate word

rubocop/cop/README.md

@@ -0,0 +1,121 @@
+# Custom Cops
+
+This folder contains custom RuboCop rules (cops) we've writen
+to enforce in-house conventions. To add a new rule, create
+a Ruby file within the directory that matches the department
+(category in RuboCop speak). Custom cops also need tests in `spec/rubocop`.
+
+## cucumber
+
+### regex_step_name
+Checks that Cucumber step definitions use Cucumber expressions
+instead of Regex. _Note:_ this may not always be possible, and this
+cop is not smart enough to detect those cases.
+
+Avoid
+```ruby
+Given /foobar/ do
+  ...
+end
+When /baz/ do
+  ...
+end
+Then /oops(\w+)/ do |it|
+  ...
+end
+```
+
+Prefer
+```ruby
+Given "foobar(s)" do
+  ...
+end
+When "baz" do
+  ...
+end
+Then "oops{str}" do |it|
+  ...
+end
+
+# Exception: sometimes we still need regex
+When /^I do the (\d+)(?:st|nd|rd|th) thing$/ do |ordinal| # rubocop:disable Cucumber/RegexStepName
+  ...
+end
+```
+
+## i18n
+
+### default_translation
+Checks for uses of the `default` keyword argument within Rails translation helpers.
+
+Avoid
+```ruby
+t(".translation_key", default: "English text")
+```
+
+Prefer
+```ruby
+# assuming the translation is in a view, the key must be defined in config/locales/views/en.yml
+t(".translation_key")
+```
+
+### deprecated_helper
+Checks for uses of the deprecated helper function, `ts`.
+Strings passed to it cannot be translated, and all calls
+will need to be replaced with `t` to enable UI translations
+in the future.
+
+Avoid
+```ruby
+ts("This will only be in English :(")
+ts("Hello %{name}", name: "world")
+```
+
+Prefer
+```ruby
+t(".relative.path.to.translation")
+t(".greeting", name: "world")
+```
+
+### deprecated_translation_key
+Checks for uses of translation keys that have been superseded
+by others or different methods of translation.
+
+Avoid
+```ruby
+Category.human_attribute_name("name_with_colon", count: 1)
+t(".relative.path.name_with_colon", count: 2)
+```
+
+Prefer
+```ruby
+Category.human_attribute_name("name", count: 1) + t("mailer.general.metadata_label_indicator")
+metadata_property(t(".relative.path.name", count: 2)) # views only
+```
+
+[Deprecated translation keys](https://github.com/otwcode/otwarchive/blob/master/.rubocop.yml#L23)
+
+## migration
+
+### large_table_schema_update
+Checks that migrations updating the schema of large tables,
+as defined in the configuration, do so safely. As of writing,
+this involves utilizing the `uses_departure!` helper.
+
+Avoid
+```ruby
+class ExampleMigration < ActiveRecord::Migration[6.1]
+  add_column :users, :new_field, :integer, nullable: true
+end
+```
+
+Prefer
+```ruby
+class ExampleMigration < ActiveRecord::Migration[6.1]
+  uses_departure! if Rails.env.staging? || Rails.env.production?
+
+  add_column :users, :new_field, :integer, nullable: true
+end
+```
+
+[Tables that require Departure](https://github.com/otwcode/otwarchive/blob/master/.rubocop.yml#L81)