[YUNIKORN-890] Update Yunikorn docs about e2e testing (#89)

Add explanation that REST interface tests are part of the e2e tests stored
in the k8shim.

docs/developer_guide/build.md

@@ -176,3 +176,9 @@ You can also use the same approach to run the scheduler locally but connecting t
 as long as the `$HOME/.kube/config` file is pointing to that remote cluster.
 
 
+## Verify external interface changes with e2e tests
+
+Yunikorn has an external REST interface which is validated by end-to-end tests. However, the tests exist in the k8shim repository.
+Whenever a change is made to the external interface, make sure that it is validated by running e2e tests or adjust the test cases accordingly.
+
+How to run the tests locally is described [here](https://github.com/apache/incubator-yunikorn-k8shim/blob/master/test/e2e/README.md).