cmd: document exit codes

README.md

@@ -54,3 +54,10 @@ Record #2 has error: wrong number of fields in line
 $ csvlint perfect.csv
 file is valid
 ```
+
+### Exit codes
+
+`csvlint` uses three different exit codes to mean different things:
+  * 0 - the file is valid
+  * 1 - couldn't parse the entire file
+  * 2 - could parse the file, but there were lint failures

cmd/csvlint/main.go

@@ -64,5 +64,5 @@ func main() {
 		fmt.Println("\nunable to parse any further")
 		os.Exit(1)
 	}
-	os.Exit(0)
+	os.Exit(2)
 }