[Book] link to the bc promise in the stable API description

Author: xabbuh

book/stable_api.rst

@@ -18,6 +18,10 @@ in the stable API is in order to fix a security issue.
 The stable API is based on a whitelist, tagged with `@api`. Therefore,
 everything not tagged explicitly is not part of the stable API.
 
+.. tip::
+
+    Read more about the stable API in :doc:`/contributing/code/bc`.
+
 .. tip::
 
     Any third party bundle should also publish its own stable API.