Add information about testing

CONTRIBUTING.md

@@ -1,5 +1,14 @@
 For now, these are just some tips:
 
+## Writing tests
+
+Contributions should include unit tests. In general, these should go in [src/test/java/com/spertus/jacquard](https://github.com/espertus/jacquard/tree/main/src/test/java/com/spertus/jacquard). There are a few exceptions...
+
+To test a method that would ordinarily be private, annotate it with the tag `@VisibleForTesting`, make the method package private, and put the test in the same package. [An example will be added soon.]
+
+If you are testing `JUnitTester`, put your test in the ordinary location, but put any tests it runs dynamically in [`src/test/java/com/spertus/jacquard/junittester`](https://github.com/espertus/jacquard/tree/main/src/test/java/com/spertus/jacquard/junittester) or a subdirectory,
+tagging the class `"IndirectTest"`. For example, see [`JUnitTesterTest.java`](https://github.com/espertus/jacquard/blob/main/src/test/java/com/spertus/jacquard/JUnitTesterTest.java), which dynamically runs [`SampleTest.java`](https://github.com/espertus/jacquard/tree/main/src/test/java/com/spertus/jacquard/junittester/SampleTest.java).
+
 ## Running Tests
 
 The `test/java` sourceset consists mostly of tests that should be directly run; however,