document how to run tests

README.md

@@ -89,6 +89,9 @@ cmake --build build
 cmake --build build -t pypangolin_pip_install
 
 # Run me some tests! (Requires Catch2 which must be manually installed on Ubuntu.)
+cmake -B build -G Ninja -D BUILD_TESTS=ON
+cmake --build build
+cd build
 ctest
 ```