Add section on #[enumcapsulate(discriminant(nested = …))] variant attribute to "README.md" file

Author: regexident

macros/README.md

@@ -279,6 +279,33 @@ enum Enum {
 }
 ```
 
+##### `#[enumcapsulate(discriminant(nested = …))]`
+
+Specify a nested sub-discriminant type for the generated discriminant variant.
+
+> [!IMPORTANT]
+> This attribute is only recognized by the `VariantDiscriminant` derive macro.
+
+If you wish a discriminant variant generated by the `VariantDiscriminant`
+derive macro to be nested, then you can do so by use
+of an `#[enumcapsulate(discriminant(nested = …))]` attribute:
+
+```rust
+#[derive(VariantDiscriminant)]
+enum VariantA {
+    VariantA1,
+    VariantA2,
+}
+
+#[derive(VariantDiscriminant)]
+enum Enum {
+    #[enumcapsulate(discriminant(nested = VariantADiscriminant))]
+    VariantA(i32),
+    
+    VariantB { b: u32 },
+}
+```
+
 ## Generics
 
 There is limited support for generic enums: