Added a description to the README that this library needs GMP

Author: waltertamboer

README.md

@@ -7,7 +7,8 @@
 [![Quality Score][ico-code-quality]][link-code-quality]
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-A PHP library to work with big integers.
+A PHP library to work with big integers. This library makes use of the GMP extension to
+do its calculations.
 
 ## Install