Add the demo server and release notes

docs/demo.rst

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+Demonstration server
+====================
+
+The demo server is for demonstration and test purposes only. Please be
+respectful and keep the demo server functional for others to be able to try it
+out.
+
+The server is reset every day at 3am, french time. If you find the server is
+unusable, you can still ask for someone to reset it manually on our Matrix
+chat channel. Please do not open tickets everytime the server is down. Please
+do not open tickets if the server is quite slow: it *is* slow because the
+machine is a cheap leased server.
+
+Keep in mind that the demo server is also used for some automated tests and runs
+the latest unstable version. If you find actual bugs when using the demo
+server, please report these!
+
+Connecting to the server
+------------------------
+
+ * Server name : ``test.mailu.io``
+ * IP address : ``51.15.169.20``
+ * Webmail : https://test.mailu.io/webmail/
+ * Admin UI : https://test.mailu.io/admin/
+ * Admin login : ``admin@test.mailu.io``
+ * Admin password : ``letmein``
+
+Adding domains
+--------------
+
+If you wish to add new domains to the server for test purposes, you could
+either direct the MX for one of you domains to ``test.mailu.io`` or have your
+MX  point to the server's IP address.
+
+Also, all subdomains of ``test.mailu.io`` point to this server. Thus, you can
+simply add ``foo.test.mailu.io`` and ``bar.test.mailu.io`` for your tests.