Add contributing guidelines for pull requests

Added guidelines for submitting pull requests in English and Chinese.

Author: ikesnowy

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Before submitting your PR
+
+- DON'T make PRs for style changes or just typos.
+- DON'T surprise us with big pull requests. Instead, file an issue and start a discussion so we can agree on a direction before you invest a large amount of time.
+- DON'T add API additions without filing an issue and discussing with us first.
+- DO write tests for your code.
+- DO write XML comments for your public APIs.
+- DO give priority to the current style of the project or file you're changing.
+
+# 提交 PR 之前
+
+- 拒绝 PR 突袭。在添加新的 API 之前，请先提 Issue 与我们讨论。
+- 只包含代码风格改变/修改错字的 PR 会被拒绝，请提 Issue。
+- 拒绝超大 PR。请确保已经在 Issue 里进行了充分讨论并达成了一致。
+- 为你的代码添加测试。如果是 BUG 修复，尽可能添加一个回归测试。
+- 为公开 API 添加 XML 注释，注释内容需要是英文的。
+- 尊重原有文件或项目的代码风格。