updated readme

README.md

@@ -153,6 +153,19 @@ fn main() {
 }
 ```
 
+In cases where you don't want a generic field to be able to change the generic type you
+can annotate it with `keep_type`.
+
+```rust
+#[derive(DefaultBuilder)]
+struct SomeOptions<T> {
+    #[builder(keep_type)]
+    the_field: T,
+}
+```
+
+this will generate a standard builder method as if `T` wasn't generic.
+
 ### Related Work
 
 For more general purposes please check out the much more powerful