Hi Gerd and Carsten:
Well, as soon as I finish writing it, we'll post that it's available. An app note for the controller board has been needed for a while, as not that many people are good at just taking a schematic and being able to use the board for something. The problem, of course, is that somebody needs to sit down and write that app note, and we're all busy with dozens of other projects.
As for the humidity sensor, the slot has been in there since the very first 2.5 controller boards, since we were thinking way back then of putting leak detection into the software based upon the humidity in the E-tube. The problem is that the sensor is relatively expensive, and it didn't make the cut for inclusion into the standard production board. It's easy enough to add yourself at home if you're handy with soldering, but you're going to have to write your own Arduino software to read it and report the results up to the BBB. At some point we will do some more work on adding flood detection to the stock software, but its not there yet.
As for the tweak that Brian refers to, if you look at the schematic for Controllers 2.5, 2.6, and 2.7 you'll see that I grounded the mounting lug for the humidity sensor. It turns out that the mounting lug is actually attached to the +V pin of the sensor. So if you just mount the sensor on the PC board, you will short the +5V supply to ground. So if you're going to use the sensor, you need to take an X-acto knife and cut the trace that goes from the solder pad of the sensor mounting lug to a nearby via, and from there down to the ground plane.
The next release of the Controller board (2.7A) will fix this issue.
Hope that helps.