Add word of advise and stability badge

[lauraluiz]

The sentence is a first draft and can be improved for sure, so please suggest. I added the experimental badge, but referring only to the project setup. When I refer to project setup I mean all the building process via Grunt, which compiles Sass into CSS, optimizes and fingerprints images, builds a static web site to check design, watches for changes, etc.

[lauraluiz]

@hajoeichler check the new text

[lauraluiz]

🎉