Document maintanance of tc39 (#3926)

* Document maintenance of tc39

---------

Co-authored-by: Mihail Stoykov <[email protected]>

js/tc39/README.md

@@ -7,15 +7,21 @@ Ways to use it:
    that was tested with this module
 2. Run `go test &> out.log`
 
-The full list of failing tests, and the error, is in `breaking_test_errors-*.json`. All errors list there with the corresponding error will *not* be counted as errors - this is what the test expects, those specific errors.
-Due to changes to Sobek it's common for the error to change, or there to be now a new error on the previously passing test, or (hopefully) a test that was not passing but now is.
-In all of those cases `breaking_test_errors-*.json` needs to be updated. Run the test with `-update` flag to update: `go test -update`
-
-NOTE: some text editors/IDEs will try to parse files ending in `json` as JSON, which given the size of `breaking_test_errors-*.json` might be a problem when it's not actually a JSON (before the edit). So it might be a better idea to name it something different if editing by hand and fix it later.
+The full list of failing tests, and the error, is in `breaking_test_errors-*.json`. All errors list there with the corresponding error will *not* be counted as errors - this is what the test expects, those specific errors. See reasons for this at the end of this document.
 
 This is a modified version of [the code in the original goja
 repo](https://github.com/dop251/goja/blob/master/tc39_test.go) that Sobek was forked from.
 
+## Maintaining the tests
+
+There are few things to keep in mind when maintaining the tests.
+
+* For most of the time, we aim to have the same version (commit hash) of the test suite as the one that [uses Sobek](https://github.com/grafana/sobek/blob/main/.tc39_test262_checkout.sh#L3). So if Sobek brings a new version of the test suite, we should update the version of the test suite in this package as well.
+* The new version of Sobek could bring a new functionality, so you might need to re-evaluate the `featuresBlockList`, `skipList` and any other list in the `tc39_test.go` file. The way to go is also keeping this lists closer to the Sobek, however they are not the same, so you might need to adjust them.
+* Due to changes to Sobek it's common for the error to change, or there to be now a new error on the previously passing test, or (hopefully) a test that was not passing but now is.
+In all of those cases `breaking_test_errors-*.json` needs to be updated. Run the test with `-update` flag to update: `go test -update`.
+* Important to mention that there should be a balance between just updating the list of the errors and actually fixing errors. So it's recommended to check the diff of the `breaking_test_errors-*.json` and case by case decide if the error should be updated or the test should be fixed.
+
 
 ## Reasons for recording breaking_test_errors.json