First use documentation

[paganaye]

Would you mind to add some documentation in the README.md file so that new users can run the tests on this repository.
I can see there is a submodule "src/test/resources/test262" wich can be fetched using this command:

git submodule update --recursive --remote

Then I fail to run it and I can't any info for it.

Some of the tests fail with
Exception in thread "main" java.io.FileNotFoundException: ./demo/index.mjs (No such file or directory)
I can't find this file