You are right, there is no rain sensor connector on board. There is not much room to put another connector, and also the ultimate goal is to make use of online weather data to detect rain.
If you want to add a rain sensor connector, you can use any of the available GPIO pins. Just connect the rain sensor (which has two wires) between teh GPIO and GND. Make sure to set the pin to input, and you may want to add a pull-up resistor if the GPIO pins do not have internal pull-up.