Add contributing.md

Author: ssx

CONTRIBUTING.md

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+# Contribution Guide
+
+This page contains guidelines for contributing to the this project. Please
+review these guidelines before submitting any pull requests.
+
+## Pull Requests
+
+The pull request process differs for new features and bugs. Before sending a
+pull request for a new feature, you should first create an issue with
+`[Proposal]` in the title. The proposal should describe the new feature, as well
+as implementation ideas. The proposal will then be reviewed and either approved
+or denied. Once a proposal is approved, a pull request may be created
+implementing the new feature. Pull requests which do not follow this guideline
+will be closed immediately.
+
+Pull requests for bugs may be sent without creating any proposal issue. If you
+believe that you know of a solution for a bug that has been filed on GitHub,
+please leave a comment detailing your proposed fix.
+
+### Feature Requests
+
+If you have an idea for a new feature you would like to see added, you may
+create an issue on GitHub with `[Request]` in the title. The feature request
+will then be reviewed.
+
+## Coding Guidelines
+
+This project follows the PSR-0, PSR-1, and PSR-2 coding standards.
+