Document on how to correctly use it for discrete value intervals (including single value intervals)

[Thomas131]

I recently got this error:
ValueError: IntervalTree: Null Interval objects not allowed in IntervalTree: Interval(123, 123)

What I wanted to do is - let's say I have this intervals: 120-122, 123, 124-130 - I wanted to merge such intervals automatically to 120-130.

I think there should be more documentation on how to best use IntervalTree for descrete values. For my current project, I think I try another approach that doesn't include intervals.

Somehow related to #59