carbon frame chipped today while I was riding I lost control of my bike; it was nothing serious, I even fell "standing", but my bike fell on a rock and the carbon frame was chipped.

Should I worry about the damage?

I read posts about using nail polish to "cover" the damage, would you advice that?

Thank you a lot!

The damaged part is the top tube, close to the handlebars.

Here is a photo of the damage:
As far as I can see from your photo, the damage to your bike caused the paint to fleck but the underlying carbon fiber looks intact. Doing anything anything abrasive would risk damaging the fibers. If your bike is under warranty, I'd ask the manufacturer. If not, I'd carefully paint over the chips so as to seal it back up and prevent further damage (physical and UV) to the underlying carbon fiber.

If your bike is of recent make, the manufacturer or your local bike store (LBS) can get touch up paint in the right color. Again, you might want to inquire with the manufacturer what you need to do to maintain your frame warranty.

If you can't find the exact color through your LBS or the manufacturer, you can go to an auto parts shop and look at their array of touch up paints and get something near enough. This might be difficult as the chips are in an area where the color is fading to black.  A color that is too light will generally be less noticeable than one that is too dark. 

While you're at the parts store, get some clear coat as well.  The little touchup paint bottles should cost about 3-8 Eurodollars each and come with their own little brushes. 

Now, with metal frames and when you have an exact paint match, you want to sand down the metal so that you can remove any oxidation and lay down some primer. But you have a carbon frame, the damaged area is so small, and sanding may cause more fibers to abrade so I wouldn't risk sanding it.

Instead, I'd clean the area with some isopropyl alcohol and then apply the touch up paint very carefully. The area is so small that you likely won't need primer either but if you got some, it wouldn't hurt. Thin layers applied multiple times with time in between to dry is better than a blob of paint. 

After the final layer had dried, apply the clear coat. 

Ps. Don't use clear nail polish lacquer as it yellows under UV and age and is quite brittle. It's only designed to last a few weeks at most and then its removed and reapplied.