Add doc for ServiceResponse

doc/development/reusing_abstractions.md

@@ -127,6 +127,42 @@ Everything in `lib/api`.
 
 Everything that resides in `app/services`.
 
+#### ServiceResponse
+
+Service classes usually have an `execute` method, which can return a
+`ServiceResponse`. You can use `ServiceResponse.success` and
+`ServiceResponse.error` to return a response in `execute` method.
+
+In a successful case:
+
+``` ruby
+response = ServiceResponse.success(message: 'Branch was deleted')
+
+response.success? # => true
+response.error? # => false
+response.status # => :success
+response.message # => 'Branch was deleted'
+```
+
+In a failed case:
+
+``` ruby
+response = ServiceResponse.error(message: 'Unsupported operation')
+
+response.success? # => false
+response.error? # => true
+response.status # => :error
+response.message # => 'Unsupported operation'
+```
+
+An additional payload can also be attached:
+
+``` ruby
+response = ServiceResponse.success(payload: { issue: issue })
+
+response.payload[:issue] # => issue
+```
+
 ### Finders
 
 Everything in `app/finders`, typically used for retrieving data from a database.