I couldn't find any Arduino project related with this sensor. If it is not a secret, why?
The sensor is trickey. Ull need AC to get reliable readings.
U can use two digital outputs to make ca. 1kHz sq.wave. Use a serial 50k..100k resistor to make a voltage divider. After cycling AC for a couple of secs. Read the analog voltage.
The datasheet can help you with info to calculate RH
When you see that it is very temperature dependent, you may want to use something like DHT22..
I've read that it is not so easy to work with them but simply can't believe that nobody didn't try to create some schematics knowing that it is a cheap sensor (if not the cheapest one)
Which sensor is used in DHT22 for getting humidity data?
yes, its inexpensive. Add components to do the temperature reading and calculations.
to your Q:
I guess same priciple (edit.. it is not) as the one u've got, but it contains firmware that does the temperature compensation and AD-conversion
Take a look - compare their datasheets
It wouldn't be bad to see a scheme with HR202L for educational purposes. I couldn't find anything similar with Google.
see #13 in this post
I've already seen that post before I started this thread, thank you 
The problem is that it lacks any kind of supplemental circuitry.
Someone could post its scheme with HR202L. According to knut_ny's message, more precisely from the link inside of it, that circuitry exists. It is not very important to me but could be useful for many people.