rational-numbers: Clarify that denominator can be 0 (#1856)
rational-numbers: Clarify that denominator can be 0 (#1856)

* rational-numbers: Clarify that denominator can be 0

In some languages the denominator can be 0 and therefore the track's implementation of this exercise may expect it. Stating `b != 0` in the description may be misleading if the track expects it.

* Move info to note
jmluy/x-common,exercism/x-common,exercism/x-common,jmluy/x-common,jmluy/x-common