Update README.md

Add description for ptr.ValueOrDefault

Author: elgopher

README.md

@@ -64,6 +64,8 @@ if ptr.Value(in.OptionalField) == "value" {
 }
 ```
 
+or get value by specifying the default value when in.OptionalField is nil:
+
 ```go
 v := ptr.ValueOrDefault(in.OptionalField, "defaultValue")
 ```
@@ -86,4 +88,4 @@ newPointer := ptr.Copy(in.OptionalField)
 
 ```shell
 go get github.com/elgopher/ptr@latest
-```
+```