add contribution guideline

CONTRIBUTING.md

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+# CS50 Contribution Guidelines
+
+Thank you for your interest in helping improve CS50's content and tools! Before contributing, please read this document in its entirety to prevent any complications.
+
+> **Note:** This repository and all other CS50 repositories **are not** the place to submit your completed problem sets or request help with assignments. If you’re stuck on a problem set, please consult one of our [many communities](https://cs50.harvard.edu/x/communities/).
+
+---
+
+## How to Contribute
+
+CS50 uses GitHub to host code, track issues, discuss feature requests, and accept pull requests.
+**Please do not email staff members to propose changes.**
+
+To propose an addition, edit, or report a bug:
+
+1. **Fork** the applicable repository (e.g., `cs50/check50`, `cs50/submit50`, `cs50/problems`, etc.).
+2. **Open an Issue** to suggest a feature or report a bug using the guidelines below.
+3. Optionally, **open a Pull Request** with your proposed changes after discussing via an issue.
+
+---
+
+## Opening an Issue
+
+When opening a new issue, please use this template:
+
+**Background**
+
+A brief explanation of the problem encountered. Be specific about what feature or bug you are addressing (problem set, command line tooling, etc.)
+
+**Steps to Reproduce**
+
+Detailed, step-by-step instructions for how someone else can reproduce the issue.
+- Be as specific as possible
+- Include code or configuration that caused the error, as well as describing the context in which it occurred, as long as it does not violate CS50's [Academic Honesty policy](https://cs50.harvard.edu/x/honesty/)
+
+**Expectation vs. Result**
+
+Describe what the expected result should be, and then what actually happened. Include as much context as possible, such as error messages, code snippets that cause the issue, situations in which the bug shows up, etc.
+
+**Additional Notes / Screenshots**
+
+Include any additional information that would be helpful to the staff as they troubleshoot (e.g., why you think it happened, troubleshooting you tried, screenshots, etc.)
+
+
+## Opening a Pull Request
+
+Once you have opened an issue, or you want to propose a fix to an already existing issue, you are welcome to:
+- Fork the respective repository
+- Make your changes
+- Open a pull request, linking the issue # that this PR is addressing in the text body.
+
+Please make the PR description as comprehensive as possible, following the guideline for opening an issue.
+
+A member of the CS50 staff will review your issue/PR and evaluate as needed.
+