setting up Go version badge

Signed-off-by: Tim Bray <tbray@textuality.com>

README.md

@@ -3,6 +3,8 @@
 [![Tests](https://github.com/timbray/topfew/actions/workflows/tests.yaml/badge.svg)](https://github.com/timbray/topfew/actions/workflows/tests.yaml)
 [![codecov](https://codecov.io/gh/timbray/topfew/branch/main/graph/badge.svg)](https://codecov.io/gh/timbray/topfew)
 [![Go Report Card](https://goreportcard.com/badge/github.com/timbray/topfew)](https://goreportcard.com/report/github.com/timbray/topfew)
+[![timbray/topfew](https://img.shields.io/github/go-mod/go-version/timbray/topfew)](https://github.com/timbray/topfew)
+
 
 A program that finds and prints out the top few records in which a certain field or combination of fields occurs most frequently.