docs: add definition of terms

Author: TomokiMiyauci

README.md

@@ -8,7 +8,17 @@ Define standard HTTP middleware specifications. This is intended to increase the
 interoperability of the HTTP library's own middleware.
 
 It consists only of the web standards stack and is compatible with many
-Browsers.
+browsers.
+
+## Terms
+
+<dl>
+<dt>Upstream</dt>
+<dd>Refers to HTTP requests forwarded from the client to the server.</dd>
+
+<dt>Downstream</dt>
+<dd>Refers to the HTTP response forwarded from the server to the client.</dd>
+</dl>
 
 ## Interface
 
@@ -28,25 +38,25 @@ interface Handler {
 
 `Middleware` has the following features:
 
-- Compatible with `Handler`.
+- Compliant with [Fetch API](https://fetch.spec.whatwg.org/).
+- Compliant with `Handler`.
 
   `Handler` is a powerful interface for handling HTTP requests. The `Middleware`
   is purely an extension and compatibility with `Handler`.
 
-- It can access to the `Request`.
-- It can access the next handler.
-- It can call the next handler.
-- It can choose not to call the next handler.
-- It can access the next handler's return value (`Response`).
-- It can return `Response`.
+- It is a pure function.
+
+  `Middleware` is a pure function that returns a value. Implementations are
+  expected to have no side effects.
+- It is self-contained.
+- It can handle upstream.
+- It can handle downstream.
+- It can handle next handler.
 
-## Middleware
+## Implementation
 
-- [http-cors](https://github.com/httpland/http-cors) - Enable CORS
-- [http-log](https://github.com/httpland/http-log) - Log for request and
-  response
-- [http-etag](https://github.com/httpland/http-etag) - ETag calculate and
-  validate
+See [chain-handler](https://github.com/httpland/chain-handler) for a concrete
+implementation that can handle middleware
 
 ## License