Create a demo project

[nsantos16]

Description

We need to create a demo project to show to the open-source community how to use this library in a simple react native application.

Checklist 📝

• Add a demo folder on the repository
• Create a basic App using Expo
• Create a screen component that has a button for open a toast
• Create a component with a custom Toast
• Add a preview and description to Readme 🤓