We don't accept any GitHub pull requests to crosvm. We use
[Chromium Gerrit](https://chromium-review.googlesource.com/) for the code review
process. See
[the contribution guide](https://google.github.io/crosvm/contributing.html) for
the details.

Add pull request template to tell we use Gerrit
github: Add pull request template to tell we use Gerrit

BUG=none
TEST=none

Change-Id: I003f0084b8b057d669ec0c6160f4d8fed2871919
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3250152
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
