diff --git a/readme.md b/readme.md
index 63fdbd3..adfc0a6 100644
--- a/readme.md
+++ b/readme.md
@@ -340,6 +340,8 @@ Options when making a Sample Plate file:
     * Standard deviation size 
   * Exponential
     * Lambda value
+  * Zipf
+    * Exponent value
 * Total number of wells on the plate
 * Well populations random or fixed
   * If random, minimum and maximum population sizes