Improve contributors' experience with more documentation.

[gcruchon]

Issue and Steps to Reproduce

Documentation needed for a better help to constributors.

Versions

1.3.21

Expected

• List of supported browsers, for contributor to test with them
• More details about scripts and what they do
• Some How-Tos (like: how to make your CSS changes available in the storybook)
• Some Guidelines (test guidelines: "what to test and how", a detail of the architecture of a component)
• Some useful tools (like VM for testing on various browsers)

Actual

Contributor have to dig into the code, to understand more how things work.

[arnaudforaison]

CONTRIBUTING.md file seems to explain some of script do. Not enough ? How do more ?
Design system, have guidelines.
Tools like ... ?

[gcruchon]

If it's OK for you, you can assign this issue to me and I can submit a first version that we can all improve during the PR.