add go version notice

Author: Lightning

README.md

@@ -2,6 +2,8 @@
 
 A simple thread-safe memoizer for Go using only the standard library.
 
+Supports Go 1.18 and higher due to the use of generics.
+
 [![Reference](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](https://pkg.go.dev/github.com/LightningDev1/go-memoizer)
 [![Linter](https://goreportcard.com/badge/github.com/LightningDev1/go-memoizer?style=flat-square)](https://goreportcard.com/report/github.com/LightningDev1/go-memoizer)
 [![Build status](https://github.com/LightningDev1/go-memoizer/actions/workflows/ci.yml/badge.svg)](https://github.com/LightningDev1/go-memoizer/actions)