Add link to how to generate new SKPs when increasing the picture version

BUG=skia:2041
R=robertphillips@google.com, scroggo@google.com, reed@google.com, bsalomon@google.com

Author: rmistry@google.com

Review URL: https://codereview.chromium.org/170923003

git-svn-id: http://skia.googlecode.com/svn/trunk@13493 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index a717153..3776ff8 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -239,6 +239,9 @@
     // V18: SkBitmap now records x,y for its pixelref origin, instead of offset.
     // V19: encode matrices and regions into the ops stream
     // V20: added bool to SkPictureImageFilter's serialization (to allow SkPicture serialization)
+
+    // Note: If the picture version needs to be increased then please follow the
+    // steps to generate new SKPs in (only accessible to Googlers): http://goo.gl/qATVcw
     static const uint32_t PICTURE_VERSION = 20;
 
     // fPlayback, fRecord, fWidth & fHeight are protected to allow derived classes to