Update principles.md

Author: andyblundell

principles.md

@@ -50,7 +50,7 @@ The following practices support the principle of building quality in.
 
 **[Test automation.](practices/testing.md)** Use test-driven development: Write the tests hand in hand with the code it is testing to ensure code is easily testable and does just enough to meet the requirements.
 
-**[Protect code quality](patterns/everything-as-code.md)** to keep code easy to maintain.
+**[Protect code quality](patterns/everything-as-code.md)** to keep code easy to maintain and to [build for testability](patterns/everything-as-code.md#benefits).
 
 **Write less code.** Treat code as a liability rather than an asset: the more code, the more there is to go wrong. Incremental delivery and test driven development both help keep the codebase small and simple.