Add Stirling class to the user guide
diff --git a/src/site/apt/userguide/index.apt b/src/site/apt/userguide/index.apt
index ac76376..c7c3f67 100644
--- a/src/site/apt/userguide/index.apt
+++ b/src/site/apt/userguide/index.apt
@@ -295,6 +295,19 @@
[3, 4, 5]
+------------------------------------------+
+ The <<<Stirling>>> class can evaluate Stirling numbers of the second kind \( S(n, k) \). This is
+ the number of ways of partitioning a set of \( n \) elements into \( k \) non-empty subsets.
+ For example a set of 3 elements may be partitioned into:
+
++------------------------------------------+
+Stirling.stirlingS2(3, 1) // 1 : {{1}, {2}, {3}}
+Stirling.stirlingS2(3, 2) // 3 : {{1, 2}, {3}}, {{1, 3}, {2}} and {{1}, {2, 3}}
+Stirling.stirlingS2(3, 3) // 1 : {{1, 2, 3}}
++------------------------------------------+
+
+ The evaluation is limited by the <<<long>>> datatype and the method will raise an
+ <<<ArithmeticException>>> if the result cannot be represented.
+
Complex
The {{{../commons-numbers-complex/index.html}commons-numbers-complex}} module
