add notes on how test runner works

Signed-off-by: shmck <shawn.j.mckay@gmail.com>

docs/docs/how-it-works.md

@@ -0,0 +1,19 @@
+---
+id: how-coderoad-works
+title: How CodeRoad Works
+sidebar_label: How CodeRoad Works
+---
+
+### Running Tests
+
+In CodeRoad, the user is given a set of directions for a **task**.
+
+Each task is judged to pass (✔) or fail (✘) by the result of code tests that runs in the background. Tests can be triggered by saving a file, or by a trigger that listens to specific files for changes.
+
+![Test Flow Diagram](../images/test-flow-diagram.png)
+
+If a test fails, the first failing test name is returned to the user as a hint to identify the problem.
+
+Tests might be in another directory. Those folders or files might even be hidden from you by the tutorial creator.
+
+But where does the code for these tests come from?