Update README.md

Author: lemire

README.md

@@ -3,7 +3,7 @@
 [![codebeat badge](https://codebeat.co/badges/fbff9479-9394-40c6-b12d-8c6b666c115e)](https://codebeat.co/projects/github-com-lemire-fastintegercompression-js)
 
 This is an integer compression library in JavaScript, useful for work on indexes.
-Given an array of small integers, it produces an ArrayBuffer that uses far fewer bytes
+Given an array of small non-negative integers, it produces an ArrayBuffer that uses far fewer bytes
 than the original (using VByte compression). It assumes a modern JavaScript engine with
 typed arrays.