Add file to explain SGML linking options, for use by SGML document

writers.

diff --git a/doc/src/sgml/README.links b/doc/src/sgml/README.links
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+
+
+Linking within SGML documents can be confusing, so here is a summary:
+
+
+Intra-document Linking
+----------------------
+
+   used to link to a section without supplying text for the link;
+   text comes from the link target;  has no close tag
+   http://www.oasis-open.org/docbook/documentation/reference/html/xref.html
+
+
+   allows link text to be supplied, requires 
+   http://www.oasis-open.org/docbook/documentation/reference/html/link.html
+
+
+linkend=
+   controls the target of the link/xref
+
+endterm=
+        allows the text of the link/xref to be taken from a different 
+   link target; endterm in a  overrides the supplied text
+
+
+External Linking
+----------------
+
+   like , but uses a URL (not a document target);  requires
+   ; if no text is specified, the URL appears as the link text
+   http://www.oasis-open.org/docbook/documentation/reference/html/ulink.html
+
+url=
+   used by  to specify the URL