A hood is useful for keeping out stray light, light coming from outside of the field of view, which could otherwise cause flare in the image. A hood also protects the front lens element against objects touching it accidentally.
The filter thread is 46mm, and you can mount screw in hoods. One hood is easily available on various action sites for around US$10. It is made out of anodized aluminum, with a matte black finish to keep out stray light. You can find this by searching for, e.g., "46mm hood summicron".
This second picture shows that the inside of the hood is ribbed, for extra protection against stray light.
Front lens cap
When screwed into the filter thread of the lens, you cannot use the standard front lens cap. However, you can use a 55mm front lens cap inside the hood, as illustrated below, mounted on the Panasonic Lumix GH1 camera.
Originally made for the Leica Summicron 28mm, it is designed for a wide angle field of view. The Lumix 20mm has a field of view corresponding to a 40mm lens on a traditional film camera, which is narrower than what the hood is designed for. Hence, extra vignetting due to light loss in the corners should in theory not be a problem. Let's check this, however.
The image below consist of crops of the upper right hand corner of two pictures of the sky, taken with and without the hood mounted to the lens. Apart from the hood, the image parameters are the same.
As you can see, the amount of vignetting is roughly the same in both images, so using the hood is safe.
If you add a filter between the lens and the hood, you may need to check for vignetting again. The extra spacing of the filter will cause the hood to extend longer relative to the front lens element, and thus may cause vignetting.
The 46mm screw in hood can also be used on the Panasonic Leica DG Macro-Elmarit 45mm f/2.8 1:1 Macro lens, which shares the same filter thread diameter.
The to be launched 14mm f/2.5 pancake lens will also feature a 46mm filter thread diameter. However, with a 28mm field of view, this hood could potentially cause vignetting, so be sure to test before using it.
This lens hood adds significant bulk to the lens, contrary to the idea of a pancake lens in the first place. Another more slim option, is to use a step down ring as a lens hood.