Docs.

Add screenshots and an explanation of interception.

doc-src/interception.html

@@ -0,0 +1,38 @@
+
+__mitmproxy__'s interception functionality lets you pause an HTTP request or
+response, inspect and modify it, and then accept it to send it on to the server
+or client. Because this is an interactive function, it's only present in
+__mitmproxy__, not in __mitmdump__. 
+
+
+### 1: Set an interception pattern
+
+<img src="@!urlTo('intercept-filt.png')!@"/>
+
+We press __i__ to set an interception pattern. In this case, the __~q__ filter
+pattern tells __mitmproxy__ to intercept all requests. For complete filter
+syntax, see the [Filter expressions](@!urlTo("filters.html")!@) section of this
+document, or the built-in help function in __mitmproxy__.
+
+### 2: Intercepted conenctions are indicated with a red exclamation mark:
+
+<img src="@!urlTo('intercept-mid.png')!@"/>
+
+### 3: You can now view and modify the request:
+
+<img src="@!urlTo('intercept-options.png')!@"/>
+
+In this case, we viewd the request by selecting it, pressed __e__ for "edit"
+and __m__ for "method" to change the HTTP request method.
+
+### 4: Accept the intercept to continue
+
+<img src="@!urlTo('intercept-result.png')!@"/>
+
+Finally, we press __a__ to accept the modified request, which is then sent on
+to the server. In this case, we changed the request from an HTTP GET to to
+OPTIONS, and Google's server has responded with a 405 "Method not allowed". 
+
+
+
+