add pr template (#550)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+<!-- Demo: https://github.com/PaddlePaddle/PaddleNLP/pull/26 -->
+### PR types
+<!-- One of [ New features | Bug fixes | Function optimization | Performance optimization | Breaking changes | Others ] -->
+
+### PR changes
+<!-- One of [ Models | APIs | Docs | Others ] -->
+
+### Describe
+<!-- Describe what this PR does -->