docs: test guidelines

CONTRIBUTING.md

@@ -131,6 +131,21 @@ Specific warnings and clippy lints can be allowed when appropriate using attribu
 #[allow(clippy::float_cmp)]
 ```
 
+### Test Guidelines
+
+Heavily algorithmic code may benefit from unit tests in Rust: create a module `mod tests` conditionally compiled with `#[cfg(test)]`, and add your tests in there.
+
+Most tests are swf-based, with the swfs stored in `core/tests/swfs`. They are configured in `core/tests/regression_tests.rs`.
+
+To add a test here, create a .swf file that runs `trace()` statements. You can do this by:
+* creating a .fla file in a Flash authoring tool
+* creating a .as file in a text editor, and compiling it using `mtasc`
+    * if you create a file `test.as` with a `class Test` with a `static function main` with the code you want to run, you can compile it using `mtasc -main -header 200:150:30 test.as -swf test.swf`
+
+Run the .swf in Flash Player and create a file `output.txt` with the contents of the trace statements. Add the `output.txt`, `test.swf` and either the `test.as` or `test.fla` file to a directory under `core/tests/swfs/avm1` (or `avm2`) named after what your test tests, and add a line in `regression_tests.rs` to have Ruffle run it.
+
+Running `cargo test [your test]` will run the .swf in Ruffle and check the `trace()` output against `output.txt`.
+
 ## Commit Message Guidelines
 
 Here is a sample commit message: