Added contributing guidelines.

CONTRIBUTING.md

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+# Contributing Guidelines
+
+The following is a set of guidelines for contributing to nginx project.
+We really appreciate that you are considering contributing!
+
+## Table of Contents
+
+- [Ask a Question](#ask-a-question)
+- [Report a Bug](#report-a-bug)
+- [Suggest a Feature or Enhancement](#suggest-a-feature-or-enhancement)
+- [Open a Discussion](#open-a-discussion)
+- [Submit a Pull Request](#submit-a-pull-request)
+- [Issue Lifecycle](#issue-lifecycle)
+
+## Ask a Question
+
+To ask a question, open an issue on GitHub with the label `question`.
+
+## Report a Bug
+
+To report a bug, open an issue on GitHub with the label `bug` using the
+available bug report issue template. Before reporting a bug, make sure the
+issue has not already been reported.
+
+## Suggest a Feature or Enhancement
+
+To suggest a feature or enhancement, open an issue on GitHub with the label
+`feature` or `enhancement` using the available feature request issue template.
+Please ensure the feature or enhancement has not already been suggested.
+
+## Open a Discussion
+
+If you want to engage in a conversation with the community and maintainers,
+we encourage you to use
+[GitHub Discussions](https://github.com/nginx/nginx/discussions).
+
+## Submit a Pull Request
+
+Follow this plan to contribute a change to NGINX source code:
+
+- Fork the NGINX repository
+- Create a branch
+- Implement your changes in this branch
+- Submit a pull request (PR) when your changes are tested and ready for review
+
+Refer to
+[NGINX Development Guide](https://nginx.org/en/docs/dev/development_guide.html)
+for questions about NGINX programming.
+
+### Formatting Changes
+
+- Changes should be formatted according to the
+[code style](https://nginx.org/en/docs/dev/development_guide.html#code_style)
+used by NGINX; sometimes, there is no clear rule, in which case examine how
+existing NGINX sources are formatted and mimic this style; changes will more
+likely be accepted if style corresponds to the surrounding code
+
+- Keep a clean, concise and meaningful commit history on your branch, rebasing
+locally and breaking changes logically into commits before submitting a PR
+
+- Each commit message should have a single-line subject line followed by verbose
+description after an empty line
+
+- Limit the subject line to 67 characters, and the rest of the commit message
+to 76 characters
+
+- Use subject line prefixes for commits that affect a specific portion of the
+code; examples include "Upstream:", "QUIC:", or "Core:"; see the commit history
+to get an idea of the prefixes used
+
+- Reference issues in the the subject line; if the commit fixes an issue,
+[name it](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
+accordingly
+
+### Before Submitting
+
+- The proposed changes should work properly on a wide range of
+[supported platforms](https://nginx.org/en/index.html#tested_os_and_platforms)
+
+- Try to make it clear why the suggested change is needed, and provide a use
+case, if possible
+
+- Passing your changes through the test suite is a good way to ensure that they
+do not cause a regression; the repository with tests can be cloned with the
+following command:
+
+```bash
+git clone https://github.com/nginx/nginx-tests.git
+```
+
+- Submitting a change implies granting project a permission to use it under the
+[BSD-2-Clause license](https://github.com/nginx/nginx/blob/master/LICENSE)
+
+## Issue Lifecycle
+
+To ensure a balance between work carried out by the NGINX engineering team
+while encouraging community involvement on this project, we use the following
+issue lifecycle:
+
+- A new issue is created by a community member
+
+- An owner on the NGINX engineering team is assigned to the issue; this
+owner shepherds the issue through the subsequent stages in the issue lifecycle
+
+- The owner assigns one or more
+[labels](https://github.com/nginx/nginx/issues/labels) to the issue
+
+- The owner, in collaboration with the wider team (product management and
+engineering), determines what milestone to attach to an issue;
+generally, milestones correspond to product releases