Add some doc regarding time intervals

@elemoine & @vicb, I just tested the implementation of WMS time sliders (PR #687). It works really nicely and smoothly.

Thanks a lot, this is really great!!

I just had some trouble to understand how to configure the time interval string. It took me quite a time to find out that I had to use the ISO 8601 time duration notation.

Thus, I'm proposing a PR which gives some indications on the definition of this interval.

Feel free to close if you don't agree (it only modifies the integrator doc...)