DOC: added docs for referee extension

Author: Liudmila Mantrova

doc/src/sgml/contrib.sgml

@@ -153,6 +153,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &pg-wait-sampling;
  &plantuner; 
  &postgres-fdw;
+ &referee;
  &rum;
  &seg;
  &sepgsql;

doc/src/sgml/filelist.sgml

@@ -164,6 +164,7 @@
 <!ENTITY pgvisibility    SYSTEM "pgvisibility.sgml">
 <!ENTITY pg-wait-sampling    SYSTEM "pg-wait-sampling.sgml">
 <!ENTITY postgres-fdw    SYSTEM "postgres-fdw.sgml">
+<!ENTITY referee         SYSTEM "referee.sgml">
 <!ENTITY rum             SYSTEM "rum.sgml">
 <!ENTITY plantuner       SYSTEM "plantuner.sgml">
 <!ENTITY seg             SYSTEM "seg.sgml">

doc/src/sgml/referee.sgml

@@ -0,0 +1,31 @@
+<sect1 id="referee">
+  <title>referee</title>
+  <para>
+    <filename>referee</filename> is a <productname>&productname;</productname>
+    extension for managing quorum settings in clusters with an even number
+    of nodes configured with <link linkend="multimaster">multimaster</link>.
+    Installed on a separate system, the <filename>referee</filename>
+    extension provides a voting node to assign the quorum status to a subset
+    of nodes if only half of the cluster nodes remain connected to each other.
+  </para>
+  <para>
+    If a multi-master cluster is split into equal parts, or half of the
+    cluster nodes fail, each subset of nodes that equals half of the
+    cluster tries to access the referee.
+    The first subset that gets connected to the referee wins the voting
+    and continues working, while all the other nodes are excluded from
+    the cluster to avoid split-brain problems.
+  </para>
+
+  <para>
+    The referee node does not store any cluster data, so it is very lightweight
+    and can be located on systems with limited resources. For details on how
+    to set up and use a referee node with <filename>multimaster</filename>,
+    see <xref linkend="setting-up-a-referee">.
+  </para>
+
+  <sect2>
+    <title>Authors</title>
+    <para>Postgres Professional, Moscow, Russia</para>
+  </sect2>
+</sect1>