IGRF badge added.

README.md

@@ -2,8 +2,7 @@
 [![golangci-lint](https://github.com/proway2/go-igrf/actions/workflows/golangci-lint.yml/badge.svg)](https://github.com/proway2/go-igrf/actions/workflows/golangci-lint.yml)
 [![Go Reference](https://pkg.go.dev/badge/github.com/proway2/go-igrf.svg)](https://pkg.go.dev/github.com/proway2/go-igrf)
 [![Go Report Card](https://goreportcard.com/badge/github.com/proway2/go-igrf)](https://goreportcard.com/report/github.com/proway2/go-igrf)
-[![Go Report Card](https://goreportcard.com/badge/github.com/proway2/go-igrf)](https://goreportcard.com/report/github.com/proway2/go-igrf)
-!\[igrf badge\](https://badgen.net/static/IGRF%20model/13th/green)
+[![IGRF badge](https://badgen.net/static/IGRF%20model/13th/green)](https://www.ncei.noaa.gov/products/international-geomagnetic-reference-field)
 
 # go-igrf
 Pure Go IGRF (International Geomagnetic Reference Field). This is based on the existing `C` implementation. This package computes values for the geomagnetic field and secular variation for a given set of coordinates and date.