README: add instructions for building on Linux + CrOS
README: add instructions for building on Linux + CrOS

We have instructions for building with Docker, but it's now possible to build
for normal Linux distros too. Also add a pointer to the main CrOS developer
guide.

BUG=none
TEST=follow the instructions

Change-Id: Ic7ce498268f8057fbe90a88166017f54108d0e16
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2057747
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Commit-Queue: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

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