Added contributing guide.

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+# Contributing guidelines
+
+The following is a set of guidelines for contributing to njs.  We do
+appreciate that you are considering contributing!
+
+## Table of contents
+
+- [Getting started](#getting-started)
+- [Ask a question](#ask-a-question)
+- [Contributing](#contributing)
+- [Git style guide](#git-style-guide)
+
+
+## Getting started
+
+Check out the [Getting started](README.md#getting-started-with-nginx-javascript)
+and [njs examples](https://github.com/nginx/njs-examples) guides to get NGINX with
+njs up and running.
+
+
+## Ask a question
+
+Please open an [issue](https://github.com/nginx/njs/issues/new) on GitHub with
+the label `question`.  You can also ask a question on the NGINX mailing list,
+nginx@nginx.org (subscribe [here](https://mailman.nginx.org/mailman/listinfo/nginx)).
+
+
+## Contributing
+
+### Report a bug
+
+Please ensure the bug has not already been reported as
+[issue](https://github.com/nginx/njs/issues).
+
+If the bug is a potential security vulnerability, please report using our
+[security policy](SECURITY.md).
+
+To report a non-security bug, open an
+[issue](https://github.com/nginx/njs/issues/new) on GitHub with the label
+`bug`.  Be sure to include a title and clear description, as much relevant
+information as possible, and a code sample or an executable test case showing
+the expected behavior that doesn't occur.
+
+
+### Suggest a feature or enhancement
+
+To suggest a feature or enhancement, please create an [issue](https://github.com/nginx/njs/issues/new)
+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.
+
+> [!NOTE]
+> If you’d like to implement a new feature, please consider creating a
+> feature request issue first to start a discussion about the feature
+> before implementing it.
+
+
+### Open a pull request
+
+Fork the repo, create a branch, implement your changes, add any relevant tests,
+submit a PR when your changes are tested and ready for review.
+
+Before submitting a PR, please read the NGINX code guidelines to learn more
+about coding conventions and style.
+
+- Try to make it clear why the suggested change is needed, and provide a use
+  case, if possible.
+
+- Changes should be formatted according to the code style used by njs.
+  njs mostly follows the [NGINX coding style](https://nginx.org/en/docs/dev/development_guide.html#code_style),
+  with some minor differences. Sometimes, there is no clear rule; in such
+  cases examine how existing njs sources are formatted and mimic this style.
+
+- Submitting changes implies granting project a permission to use it under
+  an [BSD-2-Clause license](LICENSE).
+
+
+## Git style guide
+
+- Keep a clean, concise and meaningful git commit history on your branch,
+  rebasing locally and squashing before submitting a PR
+
+- In the subject line, use the past tense ("Added feature", not "Add feature");
+  also, use past tense to describe past scenarios, and present tense for
+  current behavior
+
+- Limit the subject line to 67 characters, and the rest of the commit message
+  to 80 characters
+
+- Use subject line prefixes for commits that affect a specific portion of the
+  code; examples include "Tests:", "HTTP:", or "Core:". See the commit history
+  to get an idea of the prefixes used.
+
+- Reference issues and PRs liberally after the subject line; if the commit
+  remedies a GitHub issue, [name it](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) accordingly