Create CONTRIBUTING.md

CONTRIBUTING.md

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+### Contributing Guide
+#### 1 Issue Guidelines
+
+- Issues are exclusively for bug reports, feature requests and design-related topics. Other questions may be closed directly. If any questions come up when you are using Element, please hit [Gitter](https://gitter.im/element-en/Lobby) for help.
+
+- Before submitting an issue, please check if similar problems have already been issued.
+
+#### 2 Pull Request Guidelines
+
+- Fork this repository to your own account. Do not create branches here.
+
+- Commit info should be formatted as `[File Name]: Info about commit.` (e.g. `README.md: Fix xxx bug`)
+
+- <font color=red>Make sure PRs are created to `develop` branch instead of `master` branch.</font>
+
+- If your PR fixes a bug, please provide a description about the related bug.
+
+- Merging a PR takes two maintainers: one approves the changes after reviewing, and then the other reviews and merges.