Document dm error about missing skps

site/dev/testing/testing.md

Index: site/dev/testing/testing.md
diff --git a/site/dev/testing/testing.md b/site/dev/testing/testing.md
index 4e2f3b5a39b984294e37d65f8a81ab4dadcb1789..667e822d5e52ad0162eab53e5222239851cea3bf 100644
--- a/site/dev/testing/testing.md
+++ b/site/dev/testing/testing.md
@@ -49,6 +49,10 @@ supports many test configurations, which are not all appropriate for all
 machines.  These lines are a sort of FYI, mostly in case DM can't run some
 configuration you might be expecting it to run.
 
+Don't worry about the "skps: Couldn't read skps." messages either, you won't
+have those by default and can do without them. If you wish to test with them
+too, you can [download them separately](https://skia.org/dev/testing/skps).
+
 The next line is an overview of the work DM is about to do.
 ~~~
 492 srcs * 3 sinks + 382 tests == 1858 tasks