README: mention supported PHP versions (#19)

Author: janedbal

README.md

@@ -54,3 +54,6 @@ SqlNode is an enum of all `walkXxx` methods in Doctrine's SqlWalker, so you are
 ### Implementors
 - [shipmonk/doctrine-mysql-optimizer-hints](https://github.com/shipmonk-rnd/doctrine-mysql-optimizer-hints) (since v2)
 - [shipmonk/doctrine-mysql-index-hints](https://github.com/shipmonk-rnd/doctrine-mysql-index-hints) (since v3)
+
+### Supported PHP versions
+- PHP 7.2 - 8.3