Add some documentation for ART chroot-based device testing.

Add a `README.chroot.md` Markdown file in the `test` directory
documenting ART chroot-based device testing and containing
instructions on how to use it.

This is a starting point; there should be more documentation and
automation in the future.

Test: Render `README.chroot.md` as HTML and check the output manually
Bug: 34729697
Bug: 130188425
Change-Id: Iba868a521b920c7300e3c1232b3474e69fa79512
1 file changed