Create CONTRIBUTING.md (projectdiscovery#3968)

CONTRIBUTING.md

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+# Contributing to ProjectDiscovery/Nuclei
+
+We appreciate your interest in contributing to the projectdiscovery/nuclei! This document provides some basic guidelines for contributors.
+
+## Getting Started
+
+- Always base your work from the `dev` branch, which is the development branch with the latest code.
+- Before creating a Pull Request (PR), make sure there is a corresponding issue for your contribution. If there isn't one already, please create one.
+- Include the problem description in the issue.
+
+## Pull Requests
+
+When creating a PR, please follow these guidelines:
+
+- Link your PR to the corresponding issue.
+- Provide context in the PR description to help reviewers understand the changes. The more information you provide, the faster the review process will be.
+- Include an example of running the tool with the changed code, if applicable. Provide 'before' and 'after' examples if possible.
+- Include steps for functional testing or replication.
+- If you're adding a new feature, make sure to include unit tests.
+
+## Code Style
+
+Please adhere to the existing coding style for consistency. 
+
+## Questions
+
+If you have any questions or need further guidance, please feel free to ask in the issue or PR, or [reach out to the maintainers](https://discord.gg/projectdiscovery).
+
+Thank you for your contribution!
+