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

exercises/rational-numbers/description.md

@@ -2,6 +2,12 @@
 
 A rational number is defined as the quotient of two integers `a` and `b`, called the numerator and denominator, respectively, where `b != 0`.
 
+~~~~exercism/note
+Note that mathematically, the denominator can't be zero.
+However in many implementations of rational numbers, you will find that the denominator is allowed to be zero with behaviour similar to positive or negative infinity in floating point numbers.
+In those cases, the denominator and numerator generally still can't both be zero at once.
+~~~~
+
 The absolute value `|r|` of the rational number `r = a/b` is equal to `|a|/|b|`.
 
 The sum of two rational numbers `r₁ = a₁/b₁` and `r₂ = a₂/b₂` is `r₁ + r₂ = a₁/b₁ + a₂/b₂ = (a₁ * b₂ + a₂ * b₁) / (b₁ * b₂)`.