Well, a Berliner is a sour beer. Sour beers are made sour by lactobacillus. Grain naturally has a lot of lactobacillus and other bacteria on it. So, if you do your mash, then drain directly into a fermenting vessel, the lacto should take hold and sour your beer up nicely.
Some people will do this and call it a day. Others will drain and let it sit for a few days until the sourness is in the right place, then boil to kill off the bacteria. After that you can pitch regular yeast to finish the beer off.
Alternatively, you could do a normal boil (berliner boils are normally very short, like 15-30 minutes), then pitch a normal S.cerevisiae like Kolsch or German Ale along with a lactobacillus culture. This will give you the advantage of controlling your level of sourness, what specifically is going into the beer and give you more repeatable results.
Grain does have bugs in it. If you have ever been lazy and not cleaned out the mash tun for a day or two, you know exactly what I'm talking about. The disadvantage, and possibly what makes it fun is you don't know how much is there. That means that your no boil Berliner may sour up nicely in a week one time, but may take 2 weeks to a few months next time.
Both ways work, and both ways produce a good Berliner. For a first timer, it may be a little more worry free if you boil and pitch pure cultures though.