Clean up --intensity_props docs in README

[jmuhlich]

The additional properties defined in this tool should be listed more cleanly, e.g. in a bulleted list. Something like this:

---

Additionally the follow extra properties are available:

• gini_index A number between 0 and 1, representing how unequally the signal is distributed. See https://en.wikipedia.org/wiki/Gini_coefficient .
• intensity_median Median intensity.

For example, to calculate the Gini index and median intensity for all mask regions, specify --intensity_props gini_index intensity_median.

[FloWuenne]

This Issue can be closed as it was fixed by #43 I believe!