Disable AppVeyor CI (#2455)

diff --git a/appveyor.yml b/appveyor.yml
new file mode 100644
index 0000000..180a2c9
--- /dev/null
+++ b/appveyor.yml
@@ -0,0 +1,4 @@
+# This file exists just to avoid the AppVeyor CI to trigger.
+# See https://github.com/AOMediaCodec/libavif/issues/2448.
+skip_commits:
+  message: /.*/