Update README.md re requestFileSystem deprecation/removal

[kenguest]

window.requestFileSystem has been removed/deprecated. Mention this.

Platforms affected

Motivation and Context

Description

Testing

Checklist

• [ ] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [ ] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [ ] I've updated the documentation if necessary