Add information about trait_transformer to README

Author: djkoloski

README.md

@@ -2,6 +2,22 @@
 
 Utilities for working with impl traits in Rust.
 
+## `trait_transformer`
+
+Trait transformer is an experimental crate that generates specialized versions of a base trait. For example, if you want a `Send`able version of your trait, you'd write:
+
+```rust
+#[trait_transformer(SendIntFactory: Send)]
+trait IntFactory {
+    async fn make(&self) -> i32;
+    // ..or..
+    fn stream(&self) -> impl Iterator<Item = i32>;
+    fn call(&self) -> u32;
+}
+```
+
+Which creates a new `SendIntFactory: IntFactory + Send` trait and additionally bounds `SendIntFactory::make(): Send` and `SendIntFactory::stream(): Send`. The generated sytax is still experimental, as it relies on the nightly and unstable `async_fn_in_trait`, `return_position_impl_trait_in_trait`, and `return_type_notation` features.
+
 #### License and usage notes
 
 Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or