chore: add contributing doc

Co-authored-by: Greg Jopa <[email protected]>

Author: westeezy

CONTRIBUTING.md

@@ -0,0 +1,48 @@
+# Contributing to react-paypal-js
+
+We are always looking for ways to make our modules better. Adding features and fixing bugs allows everyone who depends
+on this code to create better, more stable applications.
+Feel free to raise a pull request to us. Our team would review your proposed modifications and, if appropriate, merge
+your changes into our code. Ideas and other comments are also welcome.
+
+## Getting Started
+
+1. Create your own [fork](https://help.github.com/articles/fork-a-repo) of this [repository](../../fork).
+
+```bash
+# Clone it
+$ git clone [email protected]:me/react-paypal-js.git
+
+# Change directory
+$ cd react-paypal-js
+
+# Add the upstream repo
+$ git remote add upstream [email protected]:paypal/react-paypal-js.git
+
+# Get the latest upstream changes
+$ git pull upstream
+
+# Install dependencies
+$ npm install
+
+# Run scripts to verify installation
+$ npm run validate
+```
+
+## Making Changes
+
+1. Make sure that your changes adhere to the current coding conventions used throughout the project, indentation, accurate comments, etc.
+2. Ensure existing tests pass (`$ npm test`) and include test cases which fail without your change and succeed with it.
+
+## Submitting Changes
+
+1. Ensure that no errors are generated by ESLint (run during `$ npm test`).
+2. Commit your changes in logical chunks, i.e. keep your changes small per single commit.
+3. Locally merge (or rebase) the upstream branch into your topic branch: `$ git pull upstream && git merge`.
+4. Push your topic branch up to your fork: `$ git push origin <topic-branch-name>`.
+5. Open a [Pull Request](https://help.github.com/articles/using-pull-requests) with a clear title and description.
+6. You should follow the [Angular.js commit message guidelines](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#-git-commit-guidelines) for both your commit messages and PR titles.
+
+If you have any questions about contributing, please feel free to contact us by posting your questions on GitHub.
+
+Copyright 2020-present, PayPal, Inc. under [the Apache 2.0 license](LICENSE.txt).