update README for testing submodule

Author: tgross

README.md

@@ -52,4 +52,6 @@ When deploying to Triton the manifest expects that the `TRITON_CNS_SEARCH_DOMAIN
 
 ### Testing
 
-The `tests/` directory includes integration tests for both the Triton and Compose example stacks described above. Running `make test/triton` will run the tests in a container locally but targeting Triton Cloud. To run those tests you'll need a Triton Cloud account with your Triton command line profile set up. The test rig will use the value of the `TRITON_PROFILE` environment variable to determine what data center to target.
+The `tests/` directory includes integration tests for both the Triton and Compose example stacks described above. Build the test runner by making sure you've pulled down the submodule with `git submodule update --init` and then `make build/tester`.
+
+Running `make test/triton` will run the tests in a container locally but targeting Triton Cloud. To run those tests you'll need a Triton Cloud account with your Triton command line profile set up. The test rig will use the value of the `TRITON_PROFILE` environment variable to determine what data center to target.