github: Add yaml for mistaken-pull-closer app

ref. https://opensource.google/docs/github/disable-features/#close

BUG=none
TEST=none

Change-Id: I850718d059737c14f546d9fad438db14bfe3955d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3250156
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@google.com>

.github/mistaken-pull-closer.yml

@@ -0,0 +1,13 @@
+# `true` will close all PRs.
+filters:
+  - true
+
+# The message to post to the closed PR.
+commentBody: |
+  Thanks for your contribution!  Unfortunately, we don't use GitHub pull
+  requests to manage code contributions to this repository.  Instead, please
+  see the [contributing guide](https://google.github.io/crosvm/contributing.html)
+  which provides full instructions on how to get involved.
+
+# Whether to add a label to the closed PR.
+addLabel: false