Docs are scattered around, and README.chromeos isn't markdown. Merge to
CONTRIBUTING doc and update the relevant parts. Rename to
README.chromeos.md and make it point to the other doc.
BUG=None
TEST=read it.
Change-Id: Ib1df58e1a7c48d6faad71faffc9527da5f3218be
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3405403
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Junichi Uekawa <uekawa@chromium.org>
