update readme with testing information

prototyping/README.md

@@ -25,3 +25,21 @@ and run!
 ```
   luau-ast Examples/SmokeTest.lua | ./PrettyPrinter
 ```
+
+## Testing
+
+We have a series of snapshot tests in the `Tests/` directory. You interact with the tests using the `tests` Python script in the `prototyping` directory. To simply run the tests, run:
+
+```sh
+tests --luau-cli ../build/luau-ast --build
+```
+
+This will build the test targets and run them. Run `tests --help` for information about all the command-line options.
+
+### Adding a new test
+
+To add a new test, add it to `Tests/{SUITE_NAME}/{CASE_NAME}`. You'll need an `in.lua` file and an `out.txt` file. The `in.lua` file is the input Luau source code, while the `out.txt` file is the expected output after running `luau-ast in.lua | test_executable`.
+
+### Updating a test
+
+If you make a change to the prototype that results in an expected change in behavior, you might want to update the test cases automatically. To do this, run `tests` with the `--accept-new-output` (`-a` for short) flag. Rather than diffing the output, this will overwrite the `out.txt` files for each test case with the actual result. Commit the resulting changes with your PR.