Merge pull request #78 from OpenBMB/77-pull_request_templatemd

add PR template

PULL_REQUEST_TEMPLATE.md

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+
+<!--
+First of all, thank you for your contribution! 😄
+-->
+
+
+### 🤔 What is the nature of this change? 
+
+- [ ] New feature 
+- [ ] Fix bug / bug 
+- [ ] Style optimization 
+- [ ] Code style optimization 
+- [ ] Performance optimization 
+- [ ] Build optimization 
+- [ ] Website, documentation, demo improvements 
+- [ ] Refactor code or style 
+- [ ] Test related 
+- [ ] Solved proposed issues 
+- [ ] Other 
+
+### 🔗 Related Issue 
+
+(Describe the source of related requirements, such as the related issue discussion link. Example: close #123, close #456.)
+
+### 💡 Background or solution 
+
+(The specific problem solved.)
+
+### 📝 Changelog 
+
+(Describe changes from the user side, and list all potential break changes or other risks.)
+