Create CONTRIBUTING.md

CONTRIBUTING.md

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+For now, these are just some tips:
+
+## Running Tests
+
+The `test/java` sourceset consists mostly of tests that should be directly run; however, 
+it also contains some source files that are essentially resources for other tests. (Perhaps
+these should be moved to `resources/`. These classes, which include [SampleTest](src/test/java/com/spertus/jacquard/junittester/SampleTest.java),
+are tagged with `"IndirectTest"`, which causes them to be ignored when running the Gradle `test` task.
+
+![IntelliJ Gradle panel screenshot with the task "test" in the "verification" section 
+selected](https://github.com/espertus/jacquard/assets/661056/5f33dcb0-76ab-4e4e-80e5-bf344c7a0ab9)
+
+
+## build.gradle
+IntelliJ sometimes grays out most of `build.gradle`. If this happens, select `Reload All Gradle Projects` (the circular icon in the top left
+of the [Gradle panel screenshot](https://github.com/espertus/jacquard/assets/661056/5f33dcb0-76ab-4e4e-80e5-bf344c7a0ab9).
+[[Stack Overflow](https://stackoverflow.com/a/60207549/631051)].