Some docs about server notices

docs/server_notices.md

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+Server Notices
+==============
+
+'Server Notices' are a new feature introduced in Synapse 0.30. They provide a
+channel whereby server administrators can send messages to users on the server.
+
+They are used as part of the communication of Privacy Polices (see
+[privacy_policy.md]), however the intention is that they may also find a use
+for features such as "Message of the day".
+
+This is a feature specific to Synapse, but it uses standard Matrix
+communication mechanisms, so should work with any Matrix client.
+
+User experience
+---------------
+
+When the user is first sent a server notice, they will get an invitation to a
+room (typically called 'Server Notices', though this is configurable in
+`homeserver.yaml`). They will be **unable to reject** this invitation -
+attempts to do so will receive an error.
+
+Once they accept the invitation, they will see the notice message in the room
+history; it will appear to have come from the 'server notices user' (see
+below).
+
+The user is prevented from sending any messages in this room by the power
+levels. They also cannot leave it.
+
+Synapse configuration
+---------------------
+
+Server notices come from a specific user id on the server. Server
+administrators are free to choose the user id - something like `server` is
+suggested, meaning the notices will come from
+`@server:<your_server_name>`. Once the server notices user is configured, that
+user id becomes a special, privileged user, so administrators should ensure
+that **it is not already allocated**.
+
+In order to support server notices, it is necessary to add some configuration
+to the `homeserver.yaml` file. In particular, you should add a `server_notices`
+section, which should look like this:
+
+```yaml
+server_notices:
+   system_mxid_localpart: server
+   system_mxid_display_name: "Server Notices"
+   room_name: "Server Notices"
+```
+
+The only compulsory setting is `system_mxid_localpart`, which defines the user
+id of the server notices user, as above. `system_mxid_display_name` and
+`room_name` define the displayname of the system notices user, and of
+the notices room, respectively.