Add pgx v5 note

Author: jackc

README.md

@@ -1,6 +1,12 @@
 [![](https://godoc.org/github.com/jackc/pgtype?status.svg)](https://godoc.org/github.com/jackc/pgtype)
 ![CI](https://github.com/jackc/pgtype/workflows/CI/badge.svg)
 
+---
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+This version is used with pgx `v4`. In pgx `v5` it is part of the https://github.com/jackc/pgx repository.
+
+---
+
 # pgtype
 
 pgtype implements Go types for over 70 PostgreSQL types. pgtype is the type system underlying the