Add overview on push.

drafts/push_overview.rst

@@ -0,0 +1,31 @@
+Push Notifications
+==================
+
+Matrix supports push notifications as a first class citizen. Home Servers send
+notifications of user events to user-configured HTTP endpoints. User may also
+configure a number of rules that determine what events generate notifications.
+These are all stored and managed by the users home server such that settings can
+be reused between client apps as appropriate.
+
+Nomenclature
+------------
+
+Pusher
+  A 'pusher' is an activity in the Home Server that manages the sending
+  of HTTP notifications for a single device of a single user.
+
+Push Rules
+  A push rule is a single rule, configured by a matrix user, that gives
+  instructions to the Home Server about whether an event should be notified
+  about and how given a set of conditions. Matrix clients allow the user to
+  configure these. They create and view them via the Client to Server REST API.
+
+Push Gateway
+  A push gateway is a server that receives HTTP event notifications from Home
+  Servers and passes them on to a different protocol such as APNS for iOS
+  devices or GCM for Android devices. Matrix.org provides a reference push
+  gateway, 'sygnal'.
+
+For information on the client-server API for setting pushers and push rules, see
+the Client Server API section. For more information on the format of HTTP
+notifications, see the HTTP Notification Protocol section.