doc: add JSON acronym

Reported-by: torsten.grust@gmail.com

Discussion: https://postgr.es/m/20171024201849.1488.71071@wrigleys.postgresql.org

Author: bmomjian

doc/src/sgml/acronyms.sgml

@@ -369,6 +369,16 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>JSON</acronym></term>
+    <listitem>
+     <para>
+      <ulink
+      url="http://json.org">JavaScript Object Notation</ulink>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>LDAP</acronym></term>
     <listitem>