add pr template (istio#307)

Author: rkpagadala

mixerclient/.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+**What this PR does / why we need it**:
+
+**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
+
+**Special notes for your reviewer**:
+
+**Release note**:
+<!--  Steps to write your release note:
+1. Use the release-note-* labels to set the release note state (if you have access)
+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
+```