book: document using crosvm tools in cros tree.
book: document using crosvm tools in cros tree.

BUG=none
TEST=n/a

Change-Id: Ifee38e534fe13a148c8a57fca7aae1b95c944175
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3617162
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm