added example to readme

Author: niklastreml

README.md

@@ -3,4 +3,33 @@
 ![GitHub go.mod Go version](https://img.shields.io/github/go-mod/go-version/niklastreml/icinga-marshaler)
 [![GoDoc](https://godoc.org/github.com/NiklasTreml/icinga-marshaler?status.svg)](https://godoc.org/github.com/NiklasTreml/icinga-marshaler)
 
-Small library that provides a utility function for easily marshalling any go struct into a format that icinga2 can understand.
+Small library that provides a utility function for easily marshalling any go struct into a format that icinga2 can understand. The library follows the specification mentioned [here](https://icinga.com/docs/icinga-2/latest/doc/05-service-monitoring/#performance-data-metrics).
+
+# Example
+
+```golang
+package main
+
+func main() {
+    type Check struct {
+		BasicValue          string
+		FieldWithThresholds int64   `warn:"800" crit:"1024" min:"64" max:"2048"`
+		FieldWithCustomName float64 `icinga:"MyCustomField"`
+		EverythingTogether  int32   `icinga:"Complex" warn:"800" crit:"1024" min:"64" max:"2048"`
+	}
+
+	status := Check{
+		BasicValue:          "WARN",
+		FieldWithThresholds: 1024,
+		FieldWithCustomName: 63.5,
+		EverythingTogether:  100,
+	}
+
+	bytes := Marshal(status)
+	fmt.Println(string(bytes))
+}
+```
+
+Creates this output:
+
+`'BasicValue'=WARN 'FieldWithThresholds'=1024;800;1024;64;2048 'MyCustomField'=63.5 'Complex'=100;800;1024;64;2048`