Document assert_duck_types_match and have_matching_duck_types in README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

README.md

@@ -135,6 +135,9 @@ assert_interfaces_match [StripeProcessor, PaypalProcessor],
 This is useful if your class implements multiple interfaces, in
 which case you can write an assertion for each.
 
+If you prefer duck typing terminology, `assert_duck_types_match`
+is available as an alias.
+
 ### RSpec
 
 Require the RSpec integration in your `spec_helper.rb`:
@@ -171,6 +174,9 @@ expect([StripeProcessor, PaypalProcessor])
   .to have_matching_interfaces(methods: %i[charge refund])
 ```
 
+If you prefer duck typing terminology, `have_matching_duck_types`
+is available as an alias.
+
 #### Shared example
 
 If you prefer shared examples, register one in `spec_helper.rb`