Update pr template for more clear instruction (istio#591)

Automatic merge from submit-queue Update pr template for more clear instruction **Release note**: ```release-note NONE ``` Former-commit-id: 58e663e
WillardHu · Aug 22, 2017 · 03c8bfb · 03c8bfb
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commit 03c8bfb
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diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
@@ -4,5 +4,6 @@
 2. Enter your extended release note in the below block; leaving it blank means using the PR title as the release note. If no release note is required, just write `NONE`.
 -->
 ```release-note
+<PLEASE PUT RELEASE-NOTE HERE, PUT NONE if NO RELEASE-NOTE>
 ```