[Docs] Adds missing file with overview

doc/book/zend.http.md

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+# Zend\Http
+
+## Overview
+
+`Zend\Http` is a primary foundational component of Zend Framework. Since much of 
+what PHP does is web-based, specifically HTTP, it makes sense to have a 
+performant, extensible, concise and consistent API to do all things HTTP. In 
+nutshell, there are several parts of `Zend\Http`:
+
+* Context-less `Request` and `Response` classes that expose a fluent API for 
+introspecting several aspects of HTTP messages:
+    * Request line information and response status information
+    * Parameters, such as those found in POST and GET
+    * Message Body
+    * Headers
+* A Client implementation with various adapters that allow for sending requests 
+and introspecting responses.
+
+## Zend\Http Request, Response and Headers
+
+The Request, Response and Headers portion of the `Zend\Http` component provides 
+a fluent, object-oriented interface for introspecting information from all the 
+various parts of an HTTP request or HTTP response. The two main objects are 
+`Zend\Http\Request` and `Zend\Http\Response`. These two classes are 
+“context-less”, meaning that they model a request or response in the same way 
+whether it is presented by a client (to **send** a request and **receive** a 
+response) or by a server (to **receive** a request and **send** a response). In 
+other words, regardless of the context, the API remains the same for 
+introspecting their various respective parts. Each attempts to fully model a 
+request or response so that a developer can create these objects from a factory,
+or create and populate them manually.