docs(demo/readme): add a readme file for demo app

[fffjacquier]

The demo app could be documented in order for people using it to understand why and how to use it

Description

Created a easy to grasp README file

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Closes#66

[hpierre74]

we shouldn't provide paths and file names, it may change

[hpierre74]

one could argue styled-components IS a UI library

[hpierre74]

I think we should not be that specific about what's in there, probably a better approach would be to add some contextual info in the demo app code itself - some description on every pages would do the trick and we would change it when developing