Add contributing guide

CONTRIBUTING.md

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+# How to contribute to Granian
+
+Thank you for considering contributing to Granian!
+
+## Support questions
+
+Please don't use issues for this. Issues are the designed tool to address bugs and feature requests in Granian itself. Use our [Github Discussions](https://github.com/emmett-framework/granian/discussions) section for questions about using Granian or issues with your own code.
+
+## Reporting issues
+
+Include the following information in your post:
+
+- Describe what you expected to happen.
+- If possible, include a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) to help us identify the issue. This also helps check that the issue is not with your own code.
+- Describe what actually happened. Include the full traceback or dump if there was an exception or panic.
+- List your Python and Granian versions, also including, if possible, the OS used and the CPU architecture, and, on async protocols, the `asyncio` loop implementation used. If possible, check if this issue is already fixed in the latest releases or the latest code in the repository.
+
+## Submitting patches
+
+If there is not an open issue for what you want to submit, prefer opening one for discussion before working on a PR. You can work on any issue that doesn't have an open PR linked to it or a maintainer assigned to it. These show up in the sidebar. No need to ask if you can work on an issue that interests you.
+
+Include the following in your patch:
+
+- Use the provided formatting commands to format your code. You can setup the environment to run these commands using the instructions below.
+- Include tests if your patch add new features or alter existing behavior. Make sure the test fails without your patch.
+- Update any relevant documentation.
+
+### First time setup in your local environment
+
+- Make sure you have a GitHub account.
+- Make sure you have an updated version of `git`.
+- Configure git with your `username` and `email` if needed.
+    
+      $ git config --global user.name 'your name'
+      $ git config --global user.email 'your email'
+
+- Fork Granian to your GitHub account by clicking the [Fork](https://github.com/emmett-framework/granian/fork) button.
+- Clone your fork locally, replacing `your-username` in the command below with your actual username.
+
+      $ git clone https://github.com/your-username/granian
+      $ cd granian
+
+- Make sure you have the latest stable version of Rust language installed.
+- Make sure you have an updated version of [uv](https://github.com/astral-sh/uv).
+- Init the environment and build Granian.
+
+      $ make build-dev
+
+### Start coding
+
+- Create a branch to identify the issue you would like to work on. A good format would be using the issue number followed by a minimal description.
+
+      $ git fetch origin
+      $ git checkout -b 123-add-some-feature origin/master
+
+- Using your favorite editor, make your changes, [committing as you go](https://afraid-to-commit.readthedocs.io/en/latest/git/commandlinegit.html#commit-your-changes).
+- Use the provided formatters and linters on your changes.
+
+      $ make format
+      $ make lint
+
+- Push your commits to your fork on GitHub and [create a pull request](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request). Link to the issue being addressed with `closes #123` in the pull request description.
+
+### Running the tests
+
+Run the basic test suite with the provided command.
+
+    $ make build-dev
+    $ make test
+
+This runs the tests for the current environment, which is usually sufficient. CI will run the full suite when you submit your pull request.