chore: add CONTRIBUTING.md [skip ci]

CONRTIBUTING.md

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+# Contributing
+
+## Reporting Bugs
+
+When submitting a new bug report, please first [search](https://github.com/oclif/core/issues) for an existing or similar report & then use one of our existing [issue templates](https://github.com/oclif/core/issues/new/choose) if you believe you've come across a unique problem. Duplicate issues, or issues that don't use one of our templates may get closed without a response.
+
+## Development
+
+**1. Clone this repository...**
+
+```bash
+$ git clone git@github.com:oclif/core.git
+```
+
+**2. Navigate into project & install development-specific dependencies...**
+
+```bash
+$ cd ./core && yarn
+```
+
+**3. Write some code &/or add some tests...**
+
+```bash
+...
+```
+
+**4. Run tests & ensure they pass...**
+
+```
+$ yarn test
+```
+
+**5. Open a [Pull Request](https://github.com/oclif/core/pulls) for your work & become the newest contributor to `@oclif/core`! 🎉**
+
+## Pull Request Conventions
+
+We use [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/). When opening a pull request, please be sure that either the pull request title, or each commit in the pull request, has one of the following prefixes:
+
+- `feat`: For when introducing a new feature. The result will be a new semver minor version of the package when it is next published.
+- `fix`: For bug fixes. The result will be a new semver patch version of the package when it is next published.
+- `docs`: For documentation updates. The result will be a new semver patch version of the package when it is next published.
+- `chore`: For changes that do not affect the published module. Often these are changes to tests. The result will be _no_ change to the version of the package when it is next published (as the commit does not affect the published version).
+
+## What _not_ to contribute?
+
+### Dependencies
+
+It should be noted that our team does not accept third-party dependency updates/PRs. We use dependabot to ensure dependencies are staying up-to-date & will ship security patches for CVEs as they occur. If you submit a PR trying to update our dependencies we will close it with or without a reference to these contribution guidelines.
+
+### Tools/Automation
+
+Our core team is responsible for the maintenance of the tooling/automation in this project & we ask collaborators to kindly not make changes to these when contributing (ex. `.github/*`, `.eslintrc.json`, package.json `scripts`, etc.)