Document cmake build/run

Author: mcm001

README.md

@@ -20,4 +20,8 @@ cmake -B build -DOPENCV_ARCH=osxuniversal -DCMAKE_OSX_ARCHITECTURES="arm64;x86_6
 cmake --build build
 ```
 
+# Uncertainty testing
+
+I configure with `cmake -B cmake_build`, build with `cmake --build cmake_build` and run with `time ./cmake_build/bin/mrcal_jni_test > /dev/null`
+
 I benchmark with `perf record -F 99 -g ./cmake_build/bin/mrcal_jni_test`