Add a build action

README.md

@@ -20,6 +20,14 @@ Pass `-DCMAKE_BUILD_TYPE=Debug` or `-DCMAKE_BUILD_TYPE=Release` to
 
 This builds a CLI that can be then found in `./build/bin/katja`.
 
+### Running the tests
+
+To run the tests follow the build instructions first and then do:
+
+```sh
+ninja -C build test
+```
+
 ## Project structure
 
 The library code is in `katja/`. Public headers are under `include/`.