[java/rational-numbers] Create mentoring.md

[dsletten]

[SleeplessByte]

@exercism/java please check!

[sanderploegsma]

This is good feedback, but is actually a bug in the exercise itself. I checked the reference implementation and it turns out it uses a completely different implementation compared to the exercise instructions.

@dsletten would you be willing to submit a PR to https://github.com/exercism/java to address this? I think the reference implementation should be updated to use

Math.pow(Math.pow(base, numerator), 1.0/denominator);

And the tolerance in the tests should be lowered to the point where this implementation passes the tests.