chore: Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Setup
+
+To begin development fork repository and run `npm install` in its root folder.
+
+## When you propose a new feature or bug fix
+
+Please make sure there is an open issue discussing your contribution before jumping into a Pull Request!
+
+There are just a few situations (listed below) in which it is fine to submit PR without a corresponding issue:
+
+- Documentation update
+- Obvious bug fix
+- Maintenance improvement
+
+In all other cases please check if there's an open an issue discussing the given proposal, if there is not, create an issue respecting all its template remarks.
+
+In non-trivial cases please propose and let us review an implementation spec (in the corresponding issue) before jumping into implementation.
+
+Do not submit draft PRs. Submit only finalized work which is ready for merge. If you have any doubts related to implementation work please discuss in the corresponding issue.
+
+Once a PR has been reviewed and some changes are suggested, please ensure to **re-request review** after all new changes are pushed. It's the best and quietest way to inform maintainers that your work is ready to be checked again.
+
+## When you want to work on an existing issue
+
+**Note:** Please write a quick comment in the corresponding issue and ask if the feature is still relevant and that you want to jump into the implementation.
+
+Check out our [help wanted](https://github.com/medikoo/es6-set/labels/help%20wanted) or [good first issue](https://github.com/medikoo/es6-set/labels/good%20first%20issue) labels to find issues we want to move forward with your help.
+
+We will do our best to respond/review/merge your PR according to priority.
+
+## Writing / improving documentation
+
+Do you see a typo or other ways to improve it? Feel free to edit it and submit a Pull Request!
+
+# Code Style
+
+We aim for a clean, consistent code style. We're using [Prettier](https://prettier.io/) to confirm one code formatting style and [ESlint](https://eslint.org/) helps us to stay away from obvious issues that can be picked via static analysis.
+
+Ideally, you should have Prettier and ESlint integrated into your code editor, which will help you not think about specific rules and be sure you submit the code that follows guidelines.
+
+## Verifying prettier formatting
+
+```
+npm run prettier-check
+```
+
+## Verifying linting style
+
+```
+npm run lint
+```
+
+# Testing
+
+This package needs to work in any ES5 environment, therefore it's good to confirm it passes tests in Node.js v0.12 release.
+
+Run tests via:
+
+```
+npm test
+```