update README

Author: ringabout

README.md

@@ -1,2 +1,8 @@
 # cookies
 HTTP Cookies for Nim.
+This module provides the `Cookie` type, which directly maps to Set-Cookie HTTP response headers, and the `CookieJar` type which contains many cookies.
+
+Overview
+`Cookie` type is used to generate Set-Cookie HTTP response headers. Server sends Set-Cookie HTTP response headers to the user agent. So the user agent can send them back to the server later.
+
+`CookieJar` contains many cookies from the user agent.