Improved readme for golden tests. (#40679)

Improved readme for impeller golden tests.

Author: gaaclarke

impeller/golden_tests/README.md

@@ -18,3 +18,18 @@ Example invocation:
 
 Currently these tests are only supported on macOS and only test the Metal
 backend to Impeller.
+
+## Adding tests
+
+To add a golden image test, the `impeller_golden_tests` target must be modified
+to generate the correct image and modification to its generated `digest.json`.
+If a test case is added to [golden_tests.cc](./golden_tests.cc), for example
+"GoldenTests.FooBar", that will turn into the golden test
+"impeller_GoldenTests_Foobar" automatically if the `SaveScreenshot()` function
+is used.
+
+The examples in `golden_tests.cc` use GLFW for rendering the tests, but
+technically anything could be used.  Using the `SaveScreenshot()` function will
+automatically update the `GoldenDigest::Instance()` which will make sure that it
+is included in the generated `digest.json`. If that function isn't used the
+`GoldenDigest` should be updated manually.