docs(contributing): Add contributing guide

CONTRIBUTING.md

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+# Contributing to Create InstantSearch App
+
+Thank you for wanting to get involved in Create InstantSearch App!
+
+The goal of the package is to enable users to create InstantSearch applications quickly.
+
+## Core concepts
+
+The `create-instantsearch-app` CLI is based on the module `createInstantSearchApp(path, options?, tasks?)`.
+
+### Folder structure
+
+```
+▸ src
+  ▸ api               The source code of the module `createInstantSearchApp()`
+  ▸ cli               The source code of the CLI `create-instantsearch-app`
+  ▸ tasks             The tasks of the app generation lifecycle
+  ▸ templates         The official templates supported
+  ▸ utils             The utility functions for the package
+▸ scripts             The end-to-end tests and release scripts
+  CHANGELOG.md
+  CONTRIBUTING.md
+  LICENSE
+  README.md
+  index.js
+  package.json
+```
+
+### Creating a template
+
+When [creating a template](README.md#templates), you will need to run end-to-end tests (`yarn run test:e2e:templates`) to update all the [snapshots](https://facebook.github.io/jest/docs/en/snapshot-testing.html). These snapshots are meant to avoid regressions that can potentially happen when we unintentionally update a behavior in the source code.
+
+## Requirements
+
+- [Node](https://nodejs.org) ≥ 8
+- [Yarn](https://yarnpkg.com)
+
+## Workflow
+
+### Filing issues
+
+Creating issues, either for reporting a bug or asking for a new feature is always great. [When doing so](https://github.com/algolia/create-instantsearch-app/issues/new/choose), you'll get asked what kind of issue you want to create. Each of these templates will help you create an effective report.
+
+### Contributing to the code
+
+Code contributions are always welcome, although you should make sure to [open an issue](https://github.com/algolia/create-instantsearch-app/issues/new/choose) to notice us what you plan to do.
+
+You will need to follow these steps:
+
+- [Fork the repository](https://help.github.com/articles/fork-a-repo/)
+- Clone the project: `git clone https://github.com/algolia/create-instantsearch-app.git`
+- Install the dependencies: `yarn`
+- [Create a new branch](https://help.github.com/articles/creating-and-deleting-branches-within-your-repository/#creating-a-branch)
+  - `fix/issue-number` for a fix
+  - `feat/name-of-the-feature` for a feature
+  - `docs/what-you-changed` for documentation
+- Apply your changes
+- Run tests:
+  - `yarn lint`
+  - `yarn test`
+  - `yarn test:e2e`
+- [Create a pull request](https://help.github.com/articles/creating-a-pull-request/)
+
+We will then review your pull request!
+
+### Releases
+
+- [Generate a new GitHub token](https://github.com/settings/tokens/new) with the scope `repo` (necessary for creating GitHub releases)
+- Store this token in an [environment variable](https://en.wikipedia.org/wiki/Environment_variable) called `GITHUB_TOKEN_CISA`
+- Make sure you're on the `master` branch
+- Run commands (do not use `yarn` for releasing):
+  - `npm run release:beta` for a new beta version
+  - `npm run release` for a new stable version
+- Follow the command-line instructions
+
+## Conventions
+
+### Commits
+
+This project follows the [conventional changelog](https://conventionalcommits.org/) approach. This means that all commit messages should be formatted using the following scheme:
+
+```
+type(scope): description
+```
+
+These commits are then used to generate the [changelog](CHANGELOG.md).
+
+---
+
+Thank you for contributing!