1 files changed, 18 insertions, 0 deletions
diff --git a/README.md b/README.md
index 93f2574..f4c8d11 100644
--- a/README.md
+++ b/README.md
@@ -108,6 +108,24 @@ fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 Fetching the following IDs from foo: [1 2 3]
 ```
 
+### Custom validation
+```go
+var args struct {
+	Foo string
+	Bar string
+}
+p := arg.MustParse(&args)
+if args.Foo == "" && args.Bar == "" {
+	p.Fail("you must provide one of --foo and --bar")
+}
+```
+
+```shell
+./example
+usage: samples [--foo FOO] [--bar BAR]
+error: you must provide one of --foo and --bar
+```
+
 ### Installation
 
 ```shell