Update CHANGELOG (#25)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9880518..10727a5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## x.x.x
+
+* Throw ArgumentError if poolSize <= 0
+
 ## 1.3.6
 
 * Set max SDK version to `<3.0.0`, and adjust other dependencies.