docs: add pull request template to improve contribution process (#407)

Author: delei

.github/pull_request_template.md

@@ -0,0 +1,26 @@
+<!--
+Thanks very much for contributing to FastExcel! Please make sure that your code changes
+are covered with tests. And in case of new features or big changes
+remember to adjust the documentation.
+
+Feel free to ping committers for the review!
+
+In case of an existing issue, reference it using one of the following:
+
+Closed: #ISSUE
+Related: #ISSUE
+
+-->
+
+## Purpose of the pull request
+
+<!-- Describe the purpose of this pull request. For example: Closed: #ISSUE-->
+
+## What's changed?
+
+<!--- Describe the change below, including rationale and design decisions -->
+
+## Checklist
+
+- [ ] I have written the necessary doc or comment.
+- [ ] I have added the necessary unit tests and all cases have passed.