Add high-level overview

README.md

@@ -12,6 +12,26 @@ Outer layers {customer}
 
 ## Proposal
 
+The trait `OperationShape` models Smithy operations.
+
+```rust
+trait OperationShape {
+    type Input;
+    type Output;
+    type Error;
+}
+```
+
+A service builder sets an operation by accepting `Operation<S, L>` where `S: Service<(Op::Input, Exts), Response = Op::Output, Error = PollError | Op::Error>>`, `OperationInput: FromRequest<P, Op>`, `Exts: FromRequest<P, Op>`, `Op::Output: IntoResponse<P, Op>`, and `Op::Error: IntoResponse<P, Op>` for `Op: OperationShape`.
+
+To construct a `Operation` there are two constructors, `from_handler` which accepts a `H: Handler<Op, Exts>` and `from_service` which accepts a `S: Flattened<Op, Exts, PollError>`. The trait `Handler<Op, Ext>` is enjoyed by all closures which accept `(Op::Input, ...)` and return `Result<Op::Input, Op::Error>`. The trait `Flattened<Op, Exts, PollError>` is enjoyed by all `Service<(Op::Input, ...), Response = Op::Output, Error = PollError | Op::Error>`. Both `Handler` and `Flattened` work to provide a common interface to convert to `S: Service<(Op::Input, Exts), Response = Op::Output, Error = PollError | Op::Error>` in `Operation<S, L>`.
+
+The `UpgradeLayer<P, Op, Exts, B>` is a `Layer<S>` applied to such `S` which uses the `FromRequest<P, Op>` and `IntoResponse<P, Op>` to provide middleware taking `S: Service<(Op::Input, Exts), Response = Op::Output, Error = PollError | Op::Error>` to `S: Service<http::Request, Response = http::Response, Error = PollError>` in a protocol and operation aware way.
+
+The `Operation<S, L>::upgrade<P, Op, Exts, B>` takes `S`, applies `UpgradeLayer<P, Op, Exts, B>`, then applies the `L: Layer<UpgradeLayer::Service>`. The `L` in `Operation<S, L>` can be set by the user to provide operation specific HTTP middleware. The `Operation::upgrade` is called in the service builder `build` method and the composition is immediately `Box`'d and collected up into the protocol specific router alongside the other routes.
+
+In this way the customer can provide, for a specific operation, middleware around `S` _and_ `S` after it's upgraded to a HTTP service via `L`.
+
 ```
 Outer layers {customer}
 |- Router {runtime + codegen}