Create PULL_REQUEST_TEMPLATE.md

Signed-off-by: David Ko <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

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+#### Which issue(s) this PR fixes:
+<!--
+Use `Issue #<issue number>` or `Issue longhorn/longhorn#<issue number>` or `Issue (paste link of issue)`. DON'T use `Fixes #<issue number>` or `Fixes (paste link of issue)`, as it will automatically close the linked issue when the PR is merged.
+-->
+Issue #
+
+#### What this PR does / why we need it:
+
+#### Special notes for your reviewer:
+
+#### Additional documentation or context