add an example for environment vars with arg names

If you want to specify both of these, and if they should have different
names, then this shows you how it can be done.

README.md

@@ -120,6 +120,23 @@ $ WORKERS='1,99' ./example
 Workers: [1 99]
 ```
 
+You can also have an environment variable that doesn't match the arg name:
+
+```go
+var args struct {
+	Workers int `arg:"--count,env:NUM_WORKERS"`
+}
+arg.MustParse(&args)
+fmt.Println("Workers:", args.Workers)
+```
+
+```
+$ NUM_WORKERS=6 ./example
+Workers: 6
+$ NUM_WORKERS=6 ./example --count 4
+Workers: 4
+```
+
 ### Usage strings
 ```go
 var args struct {