[Rat] explain overflow of denominator and fallback to Num

lib/Rat.pod

@@ -0,0 +1,32 @@
+=begin pod
+
+=head1 Rat
+
+    class Rat is Cool does Rational[Int, UInt64] { ... }
+
+C<Rat> objects store rational numbers as a pair of a numerator and
+denominator. Number literals with a dot but without exponent produce
+C<Rat>s.
+
+    3.1;    # Rat.new(31, 10)
+
+That way arithmetic with short dotted-decimal numbers does not suffer
+from floating point errors.
+
+To prevent the numerator and denominator to become pathologically large,
+the denominator is limited to 64 bit storage. On overflow of the denomniator
+a C<Num> (floating-poing number) is returned instead.
+
+For example this function crudely approximates a square root, and overflows
+the denominator quickly:
+
+    sub approx-sqrt($n, $iterations) {
+        my $x = $n;
+        $x = ($x + $n / $x) / 2 for ^$iterations;
+        return $x;
+    }
+    say approx-sqrt(2, 5).WHAT;     # Rat()
+    say approx-sqrt(2, 10).WHAT;    # Num()
+
+
+=end pod