add PULL_REQUEST_TEMPLATE template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+<!--
+
+
+
+Do NOT send pull request to `master` branch.
+
+
+Please send to `dev` branch instead.
+
+
+Any PR to `master` branch will NOT be merged.
+
+
+-->