Add missing links to readme

README.md

@@ -86,6 +86,8 @@ In this example, the `register` method has a service call to its collaborators,
 
 Mockists would probably claim that testing how collaborators are called during the test process allows us to test the UserService. Let's move on to the next section and see the mockists' solution in greater detail.
 
+[link-sls-6.26.1]: https://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#value-conversions
+
 ## The Solution
 
 In this sort of situations we need mock implementations of our _collaborator service_. As _Martin Fowler_ puts it in his excellent article [Mocks Aren't Stubs][link-test-doubles]:
@@ -101,6 +103,8 @@ In this example, we can define these three test cases:
 
 ZIO Test provides a framework for mocking our modules. In the next section, we are going to test `UserService` by mocking its collaborators.
 
+[link-test-doubles]: https://martinfowler.com/articles/mocksArentStubs.html
+
 ## Mocking Collaborators
 
 In the previous section, we learned we can test the `UserService` by mocking its collaborators. Let's see how we can mock the `EmailService` and also the `UserRepository`.