Add instructions for running tests

Author: ridiculousfish

README.md

@@ -152,6 +152,16 @@ make -j
 sudo make install
 ```
 
+## Test instructions
+
+To run the test suite:
+
+```bash
+cmake -B build
+cmake --build build -j
+ctest --test-dir build
+```
+
 ## Tester program
 
 You can pass the **tester** program one or more of the following arguments: ```u32```, ```s32```, ```u64```, ```s64``` to test the four cases (signed, unsigned, 32-bit, or 64-bit), or run it with no arguments to test all four. The tester will verify the correctness of libdivide via a set of randomly chosen numerators and denominators, by comparing the result of libdivide's division to hardware division. It will stop with an error message as soon as it finds a discrepancy.