Add comments to suppressions file

No-op CL intended to trigger a rebuild.

TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1036583002

testing/SUPPRESSIONS

diff --git a/testing/SUPPRESSIONS b/testing/SUPPRESSIONS
index 2fce4e4..1d417b2 100644
--- a/testing/SUPPRESSIONS
+++ b/testing/SUPPRESSIONS
@@ -1,3 +1,5 @@
+# List of tests to be skipped, one per line.
+# Try to keep the file alphabetized.
 1_10_watermark.pdf
 1_1_textbox.pdf
 1_2_typewriter.pdf