Automatically add a docs preview link and NOTRY=true when there are only docs changes.

These changes are possible due to the recently submitted depot_tools change
https://codereview.chromium.org/949273002/ ('Add ability to specify and run post upload hooks').

BUG=skia:3474

Review URL: https://codereview.chromium.org/960203002
diff --git a/codereview.settings b/codereview.settings
index 0dba340..f8e8c9d 100644
--- a/codereview.settings
+++ b/codereview.settings
@@ -5,3 +5,4 @@
 BUG_PREFIX: skia:
 TRYSERVER_SVN_URL: https://skia-try.googlecode.com/svn
 PROJECT: skia
+RUN_POST_UPLOAD_HOOK: True