document how to make IPv6 work (#2088)

* document how to make IPv6 work

* spell out that pip will install 17.1 by default

README.rst

@@ -245,6 +245,25 @@ Setting up a TURN server
 For reliable VoIP calls to be routed via this homeserver, you MUST configure
 a TURN server.  See `<docs/turn-howto.rst>`_ for details.
 
+IPv6
+----
+
+As of Synapse 0.19 we finally support IPv6, many thanks to @kyrias and @glyph
+for providing PR #1696.
+
+However, for federation to work on hosts with IPv6 DNS servers you **must**
+be running Twisted 17.1.0 or later - see https://github.com/matrix-org/synapse/issues/1002
+for details.  We can't make Synapse depend on Twisted 17.1 by default
+yet as it will break most older distributions (see https://github.com/matrix-org/synapse/pull/1909)
+so if you are using operating system dependencies you'll have to install your
+own Twisted 17.1 package via pip or backports etc.
+
+If you're running in a virtualenv then pip should have installed the newest
+Twisted automatically, but if your virtualenv is old you will need to manually
+upgrade to a newer Twisted dependency via:
+
+    pip install Twisted>=17.1.0
+
 
 Running Synapse
 ===============