Add a simple CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Welcome to the `esp-hal` contributing guide
+
+Thank you for investing your time in contributing to our project!
+
+In this guide you will get an overview of the contribution workflow from opening an issue, creating a PR, reviewing, and merging the PR.
+
+Use the table of contents icon (<img src="resources/table-of-contents.png" width="24" height="24" />) in the top right corner of this document to get to a specific section of this guide quickly.
+
+## New Contributor Guide
+
+To get an overview of the project, please read the [README]. Here are some resources to help you get started with open source contributions:
+
+- [Finding ways to contribute to open source on GitHub]
+- [Set up Git]
+- [GitHub flow]
+- [Collaborating with pull requests]
+
+[README]: README.md
+[Finding ways to contribute to open source on GitHub]: https://docs.github.com/en/get-started/exploring-projects-on-github/finding-ways-to-contribute-to-open-source-on-github
+[Set up Git]: https://docs.github.com/en/get-started/quickstart/set-up-git
+[GitHub flow]: https://docs.github.com/en/get-started/quickstart/github-flow
+[Collaborating with pull requests]: https://docs.github.com/en/github/collaborating-with-pull-requests
+
+## Getting Started
+
+### Issues
+
+#### Create a New Issue
+
+If you spot a problem with the docs, [search if an issue already exists]. If a related issue doesn't exist, you can open a new issue using the [issue form].
+
+[search if an issue already exists]: https://docs.github.com/en/github/searching-for-information-on-github/searching-on-github/searching-issues-and-pull-requests#search-by-the-title-body-or-comments
+[issue form]: https://github.com/esp-rs/esp-hal/issues/new/
+
+#### Solve an Issue
+
+Scan through our [existing issues] to find one that interests you. You can narrow down the search using labels as filters. If you find an issue to work on, you are welcome to open a PR with a fix.
+
+It's recommended that you comment in the relevant issue, mentioning that you are actively working on it, however this is not a requirement.
+
+If somebody is already assigned to an issue, this does not necessarily mean they are actively working on it; don't be afraid to comment in these issues asking if you can take over the work if you're interested.
+
+[existing issues]: https://github.com/esp-rs/esp-hal/issues
+
+### Make Changes
+
+1. Fork the repository.
+   - Using GitHub Desktop:
+     - [Getting started with GitHub Desktop] will guide you through setting up Desktop.
+     - Once Desktop is set up, you can use it to [fork the repo!]
+   - Using the command line:
+     - [Fork the repo] so that you can make your changes without affecting the original project until you're ready to merge them.
+2. Install or update to the latest version of Rust. See [rustup.rs] for more information.
+3. Create a working branch and start with your changes!
+
+[Getting started with GitHub Desktop]: https://docs.github.com/en/desktop/installing-and-configuring-github-desktop/getting-started-with-github-desktop
+[fork the repo!]: https://docs.github.com/en/desktop/contributing-and-collaborating-using-github-desktop/cloning-and-forking-repositories-from-github-desktop
+[Fork the repo]: https://docs.github.com/en/github/getting-started-with-github/fork-a-repo#fork-an-example-repository
+[rustup.rs]: https://rustup.rs/
+
+### Commit Your Update
+
+Commit the changes once you are happy with them. Don't forget to self-review to speed up the review process.
+
+We ask that you ensure all source code files has been properly formatted with `rustfmt`, and that you have linted your changes by running `cargo clippy`. These tools can be installed by running the following commands:
+
+```shell
+rustup component add rustfmt
+rustup component add clippy
+```
+
+We _strongly_ recommend that you use the supplied `pre-commit` Git hook, which will ensure that all source code has been formatted correctly prior to committing. See the [Git documentation] for more information on hooks.
+
+The `pre-commit` hook can be installed by running the following command in a terminal, from the root of the repository:
+
+```shell
+cp pre-commit .git/hooks/pre-commit
+```
+
+[Git documentation]: https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks
+
+### Pull Request
+
+When you're finished with the changes, create a pull request, also known as a PR.
+
+- Fill the pull request template so that we can review your PR. This template helps reviewers understand your changes as well as the purpose of your pull request.
+- Don't forget to [link PR to issue] if you are solving one.
+- Enable the checkbox to [allow maintainer edits] so the branch can be updated for a merge. Once you submit your PR, a Docs team member will review your proposal. We may ask questions or request additional information.
+- We may ask for changes to be made before a PR can be merged, either using [suggested changes] or pull request comments. You can apply suggested changes directly through the UI. You can make any other changes in your fork, then commit them to your branch.
+- As you update your PR and apply changes, mark each conversation as [resolved].
+- If you run into any merge issues, checkout this [git tutorial] to help you resolve merge conflicts and other issues.
+
+[link PR to issue]: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue
+[allow maintainer edits]: https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork
+[suggested changes]: https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request
+[resolved]: https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/commenting-on-a-pull-request#resolving-conversations
+[git tutorial]: https://github.com/skills/resolve-merge-conflicts
+
+### Your PR is Merged!
+
+Congratulations! The esp-rs team thanks you for your contributions!