Documentation request for X² test

### Description

A proper explanation/tutorial for handling chi-squared-tests is missing. There seems to be at least [three versions of the Chi-Square test](https://de.wikipedia.org/wiki/Chi-Quadrat-Test) existing:
- distribution test
- independence test
- homogenity test

A description is needed that introduces the core concept of the X²-distributed test statistic and how to apply the tests.

@omaus started to look at the test in #181 but I think there is some sort of proper description missing for which test requires which parameters etc.

Thanks for pointing this out @matthewcrews :rocket: