Add proxy example (#225)

* Add proxy example

* Updated after review comments

proxy-blocker/README.md

@@ -0,0 +1,22 @@
+# proxy-filter
+
+## What it does
+
+This add-on registers a [PAC script](https://developer.mozilla.org/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling/Proxy_Auto-Configuration_%28PAC%29_file) using the proxy API.
+
+The PAC script is initialized with a list of hostnames: it blocks requests to any hosts that are in the list.
+
+The list is given the following default values: `["example.com", "example.org"]`, but the user can add and remove hosts using the add-on's options page.
+
+Note that the hostname-matching is very simple: hostnames must exactly match an entry in the list if they are to be blocked. So with the default settings, "example.org" would be blocked but "www.example.org" would be permitted.
+
+To try it out:
+* install it
+* try visiting `http://example.com`, and see it is blocked
+* visit `about:addons`, open the add-on's preferences, and try changing the hostnames in the text box
+* try visiting some different pages, to see the effect of your changes.
+
+## What it shows
+
+* How to implement a simple PAC script, and register it using the proxy API.
+* How to exchange messages between a PAC script and a background script.