docs(contributing): add CONTRIBUTING.md [ci skip]

Closes #5

CONTRIBUTING.md

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+# Contributing
+
+## Getting Started
+
+### Prerequisites
+
+This project uses node (>=10) and yarn (>=1).
+
+- [node](https://nodejs.org/en/download/package-manager/#macos) (version 10.x or greater)
+- [yarn](https://yarnpkg.com/en/docs/install#mac-stable) (version 1.x or greater)
+
+### Fork and Clone
+
+Fork the repo and clone your fork:
+
+```bash
+git clone <YOUR_FORK>
+cd carbon-components-svelte
+```
+
+Set the original repo as the upstream:
+
+```bash
+git remote add upstream git@github.com:IBM/carbon-components-svelte.git
+# verify that the upstream is added
+git remote -v
+```
+
+### Install
+
+Install the project dependencies:
+
+```bash
+yarn install
+```
+
+---
+
+## Workflow
+
+### Repo Structure
+
+```js
+.storybook // Storybook configuration
+docs // static, exported Storybook used by GitHub Pages
+src
+│
+└───components // individual components
+└───internal // code excluded from the component library
+└───lib // shared utilities
+```
+
+### Developing
+
+This project uses Storybook for UI development and "live" documentation.
+
+Run the following command to start the Storybook:
+
+```bash
+yarn start -p 9090
+```
+
+#### Component Format
+
+Individual components are located in the `src/components` folder:
+
+Each component should adopt the following structure:
+
+```js
+src/components/Component
+│
+└───Component.svelte // main component
+└───Component.Skeleton.svelte // Skeleton component (if any)
+└───Component.Story.svelte // wrapper for individual stories
+└───Component.stories.js // Storybook stories
+└───index.js // export components (e.g. `Component.svelte`, `Component.Skeleton.svelte`)
+```
+
+### Building
+
+#### Component Library
+
+To build the component library, run `yarn build`.
+
+The library should be compiled in two formats:
+
+- **ES**: `lib/index.mjs`
+- **UMD**: `lib/index.js`
+
+#### Storybook
+
+To build the Storybook, run `yarn build:storybook`.
+
+The Storybook should be outputted to the `docs` folder.
+
+## Submitting a Pull Request
+
+### Sync Your Fork
+
+Before submitting a pull request, make sure your fork is up to date with the latest upstream changes.
+
+```bash
+git fetch upstream
+git checkout master
+git merge upstream/master
+```
+
+### Submit a PR
+
+After you've pushed your changes to remote, submit your PR. Make sure you are comparing `<YOUR_USER_ID>/feature` to `origin/master`.