Add Ruby snippet for calculating compound interest

Author: ACR1209

snippets/ruby/math-and-numbers/calculate-compound-interest.md

@@ -0,0 +1,17 @@
+---
+title: Calculate Compound Interest
+description: Calculates compound interest for a given principal amount, rate, and time period.
+author: ACR1209
+contributors: axorax
+tags: ruby,math,compound interest,finance
+---
+
+```ruby
+def compound_interest(principal, rate, time, n = 1)
+    principal * (1 + rate / n) ** (n * time)
+end
+
+# Usage:
+puts compound_interest(1000, 0.05, 5)   # Output: 1276.2815625000003
+puts compound_interest(1000, 0.05, 5, 12) # Output: 1283.3586785035118
+```