Add special Windows instructions for getting SKPs

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2184723002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2184723002

Review-Url: https://codereview.chromium.org/2184723002
diff --git a/site/dev/testing/skps.md b/site/dev/testing/skps.md
index 716f1c6..2d890de 100644
--- a/site/dev/testing/skps.md
+++ b/site/dev/testing/skps.md
@@ -37,6 +37,7 @@
     $ infra/bots/tools/luci-go/${PLATFORM}/cipd auth-login
     $ infra/bots/assets/skp/download.py -t ${TARGET_DIR}
 
+Note: If you are on Windows, you will need to specify *cipd.exe.sha1*, not *cipd.sha1*
 
 <a name="buildbot_skps_partners"></a>
 ### Buildbot SKPs (for trusted partners)