Add Contributing.md (#140)

Author: NickTomlin

.changeset/tender-rocks-smell.md

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+---
+"codemirror-json-schema": patch
+---
+
+Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+## Your first contribution
+
+### Setting up the repository
+
+1. Fork this repository
+2. Create a new feature branch
+
+```bash
+git checkout -b my-feature-branch
+```
+
+### Setting up a development environment
+
+1. Ensure that you have `Node.js` installed on your machine (you can use [fnm](https://github.com/Schniz/fnm) or [nvm](https://github.com/nvm-sh/nvm) to easily manage versions)
+2. Install the dependencies with [pnpm](https://pnpm.io/installation) :warning: yarn or npm aren't supported :warning:
+
+- You can check the `packageManager` field of [`package.json`](https://github.com/jsonnext/codemirror-json-schema/blob/main/package.json#L26) to know what version of pnpm to install
+
+```bash
+pnpm install
+```
+
+3. Run tests to ensure your project is in a good state
+
+```bash
+pnpm test
+```
+
+### Making your changes
+
+1. Make whatever code changes you need
+2. Add or update tests coverage whenever possible
+
+### Verifying
+
+1. Run the tests `pnpm run test`
+2. View the changes in the demo site locally `pnpm run dev`
+3. Ensure that the project builds properly `pnpm run tsc`
+
+### Opening a pull request
+
+1. Once your branch is ready, and it contains bugfixes, documentation improvements or features, run `pnpm changeset add` to add a [changeset](https://github.com/changesets/changesets/blob/main/docs/adding-a-changeset.md) file
+   - There you can compose the markdown entry describing your changes that will be published in CHANGELOG.md
+2. Push your changes to your feature branch
+3. Open a pull request from your fork to the original repository in the Github UI or [CLI](https://cli.github.com/manual/gh_pr_create)