Add testing guide

Author: eruens

docs/testing.md

@@ -0,0 +1,13 @@
+# Testing Guide
+
+## Running Tests
+```bash
+npm test
+```
+
+## Test Structure
+- Unit tests: `tests/unit/`
+- Integration tests: `tests/integration/`
+
+## Coverage
+Aim for 80%+ coverage on all service endpoints.