docs: add php support info

Author: adhocore

README.md

@@ -10,6 +10,7 @@
 
 - Lightweight Cron expression parser library for PHP.
 - Very **fast** because it bails early in case a segment doesnt match.
+- Supports PHP5.4 (if you are old school) to PHP7.3 (if you are cool).
 - Real [benchmark](https://github.com/adhocore/php-cron-bench) shows it is about 9.64x to 10.57x faster than `dragonmantank/cron-expression`
 
 ## Installation