INSTALL: configure /etc/xml/catalog to build docs on Cygwin

Without additional configuration steps, the documentation build on Cygwin
fails because the XML catalog is missing required rewrites for certain
docbook resources.

This patch documents the required configuration.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -157,3 +157,36 @@ Issues of note:
    It has been reported that docbook-xsl version 1.72 and 1.73 are
    buggy; 1.72 misformats manual pages for callouts, and 1.73 needs
    the patch in contrib/patches/docbook-xsl-manpages-charmap.patch
+
+   Users attempting to build the documentation on Cygwin may need to ensure
+   that the /etc/xml/catalog file looks something like this:
+
+   <?xml version="1.0"?>
+   <!DOCTYPE catalog PUBLIC
+      "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN"
+      "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd"
+   >
+   <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
+     <rewriteURI
+       uriStartString = "http://docbook.sourceforge.net/release/xsl/current"
+       rewritePrefix = "/usr/share/sgml/docbook/xsl-stylesheets"
+     />
+     <rewriteURI
+       uriStartString="http://www.oasis-open.org/docbook/xml/4.5"
+       rewritePrefix="/usr/share/sgml/docbook/xml-dtd-4.5"
+     />
+  </catalog>
+
+  This can be achieved with the following two xmlcatalog commands:
+
+  xmlcatalog --noout \
+     --add rewriteURI \
+        http://docbook.sourceforge.net/release/xsl/current \
+        /usr/share/sgml/docbook/xsl-stylesheets \
+     /etc/xml/catalog
+
+  xmlcatalog --noout \
+     --add rewriteURI \
+         http://www.oasis-open.org/docbook/xml/4.5/xsl/current \
+         /usr/share/sgml/docbook/xml-dtd-4.5 \
+     /etc/xml/catalog