Create CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+Support and contributions from the open source community are essential for keeping
+`@octokit/rest.js` up to date and always improving! There are a few guidelines that we need
+contributors to follow to keep the project consistent, as well as allow us to keep
+maintaining `@octokit/rest.js` in a reasonable amount of time.
+
+Please note that this project is released with a [Contributor Code of Conduct][coc].
+By participating in this project you agree to abide by its terms.
+
+[coc]: ./CODE_OF_CONDUCT.md
+
+## Creating an Issue
+
+Before you create a new Issue:
+
+1. Please make sure there is no [open issue](https://github.com/octokit/rest.js/issues?utf8=%E2%9C%93&q=is%3Aissue) yet.
+2. If it is a bug report, include the steps to reproduce the issue and please create a reproducible test case on [runkit.com](https://runkit.com/). Example: https://runkit.com/gr2m/5aa034f1440b420012a6eebf
+3. If it is a feature request, please share the motivation for the new feature and how you would implement it.
+4. Please include links to the corresponding github documentation.
+
+## Tests
+
+If you want to submit a bug fix or new feature, make sure that all tests are passing.
+
+Before running any tests you have to start the [fixtures server](https://github.com/octokit/fixtures-server)
+
+```
+$ npm run start-fixtures-server
+```
+
+In a second terminal, run the tests
+
+```bash
+$ npm test
+```
+
+Or run a specific test
+
+```bash
+$ ./node_modules/.bin/jest test/integration/smoke.test.js
+```
+
+The examples are run as part of the tests. You can set an `EXAMPLES_GITHUB_TOKEN` environment
+variable (or set it in a `.env` file) to avoid running against GitHub's rate limit.
+
+## Making Changes
+
+Here is an overview of [how `@octokit/rest` works](HOW_IT_WORKS.md).
+
+- Create a topic branch from the main branch.
+- Check for unnecessary whitespace / changes with `git diff --check` before committing.
+- Keep git commit messages clear and appropriate. Ideally follow commit conventions described below.
+
+## Submitting the Pull Request
+
+- Push your changes to your topic branch on your fork of the repo.
+- Submit a pull request from your topic branch to the main branch on the `rest.js` repository.
+- Be sure to tag any issues your pull request is taking care of / contributing to. \* Adding "Closes #123" to a pull request description will auto close the issue once the pull request is merged in.
+
+## Testing a pull request from github repo locally:
+
+- You can install a specific pull request of a module from github using the command
+  `npm install octokit/rest.js#branchname`
+
+- For example to test pull request [#792](https://github.com/octokit/rest.js/pull/792) run `npm install octokit/rest.js#791/error-headers` as `791/error-headers` is the branch name.
+
+- After the installation is done, you can double check with `npm ls @octokit/rest` which will show something like
+
+```text
+└── @octokit/[email protected]  (git+https://github.com/octokit/rest.js.git#505ed1f57671480b625131abb00c277c67cae40a)
+```
+
+- Once you are done testing, you can revert back to the default module `@octokit/rest` from npm with `npm install @octokit/rest`
+
+## Merging the Pull Request & releasing a new version
+
+Releases are automated using [semantic-release](https://github.com/semantic-release/semantic-release).
+The following commit message conventions determine which version is released:
+
+1. `fix: ...` or `fix(scope name): ...` prefix in subject: bumps fix version, e.g. `1.2.3` → `1.2.4`
+2. `feat: ...` or `feat(scope name): ...` prefix in subject: bumps feature version, e.g. `1.2.3` → `1.3.0`
+3. `BREAKING CHANGE:` in body: bumps breaking version, e.g. `1.2.3` → `2.0.0`
+
+Only one version number is bumped at a time, the highest version change trumps the others.
+Besides publishing a new version to npm, semantic-release also creates a git tag and release
+on GitHub, generates changelogs from the commit messages and puts them into the release notes.
+
+If the pull request looks good but does not follow the commit conventions, use the "Squash & merge" button.