Simplify README
Simplify README

Since we've already moved contents of README to mdbook, let's have only a simple
description and a list of links in README.

BUG=b:199874828
TEST=none

Change-Id: I42213a0be915154309443cdd07493ef3c64842a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3159890
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

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