docs: document new feature

Author: niklastreml

README.md

@@ -30,3 +30,27 @@ func main() {
 Creates this output:
 
 `'BasicValue'=WARN 'FieldWithThresholds'=1024;800;1024;64;2048 'MyCustomField'=63.5 'Complex'=100;800;1024;64;2048`
+
+## Automatic exit codes
+You can get an exit code based on performance data and the Min,Max,Warn and Crit tags. This is useful if you want to exit your program with the correct exit code. You can use the ExitCode() function for this. It will compare the performance data to the thresholds and return the correct exit code. If the performance data is not a number, it will return 3 (UNKNOWN).
+
+```go
+package main
+
+import (
+	"os"
+)
+
+func main() {
+    type Check struct {
+		MyField int64   `warn:"800" crit:"1024" min:"64" max:"2048"`
+	}
+
+	status := Check{
+		MyField: 1200,
+	}
+
+	code := ExitCode(status)
+	os.Exit(code) // exits with code 2, i.e. CRITICAL
+}
+```

exit_code.go

@@ -7,10 +7,20 @@ import (
 	"strconv"
 )
 
+// Calculates the exit code for a given struct
+// The struct must have the following tags:
+// - warn: warning threshold
+// - crit: critical threshold
+// - min: minimum value
+// - max: maximum value
 func ExitCode(v any) (int, error) {
 	return exitCode(v, OK)
 }
 
+// Recursively walks through the struct and calculates the exit code
+// The exit code is the highest exit code of all fields
+// If a field is a struct, it will be walked through recursively
+// If a field is not a struct, it will be compared to the thresholds
 func exitCode(v any, code int) (int, error) {
 	value := reflect.Indirect(reflect.ValueOf(v))
 	log.Printf("%+v\n", reflect.DeepEqual(value, reflect.Value{}))