Initial OsmoGbPROXY user manual

This adds a very basic manual consisting of nothing more than
the common chapters and a high-level description of what it is
all about.

Change-Id: I80d4ea016376c59995ccfcd8685c7c0e86745bd2
diff --git a/doc/manuals/osmogbproxy-usermanual-docinfo.xml b/doc/manuals/osmogbproxy-usermanual-docinfo.xml
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+<revhistory>
+  <revision>
+    <revnumber>1</revnumber>
+    <date>March 21, 2019</date>
+    <authorinitials>HW</authorinitials>
+    <revremark>
+      Initial version.
+    </revremark>
+  </revision>
+</revhistory>
+
+<authorgroup>
+  <author>
+    <firstname>Harald</firstname>
+    <surname>Welte</surname>
+    <email>hwelte@sysmocom.de</email>
+    <authorinitials>HW</authorinitials>
+    <affiliation>
+      <shortaffil>sysmocom</shortaffil>
+      <orgname>sysmocom - s.f.m.c. GmbH</orgname>
+      <jobtitle>Managing Director</jobtitle>
+    </affiliation>
+  </author>
+</authorgroup>
+
+<copyright>
+  <year>2013-2019</year>
+  <holder>sysmocom - s.f.m.c. GmbH</holder>
+</copyright>
+
+<legalnotice>
+  <para>
+	Permission is granted to copy, distribute and/or modify this
+	document under the terms of the GNU Free Documentation License,
+	Version 1.3 or any later version published by the Free Software
+	Foundation; with no Invariant Sections, no Front-Cover Texts,
+	and no Back-Cover Texts.  A copy of the license is included in
+	the section entitled "GNU Free Documentation License".
+  </para>
+  <para>
+	The Asciidoc source code of this manual can be found at
+	<ulink url="https://git.osmocom.org/osmo-sgsn/doc/">
+		https://git.osmocom.org/osmo-sgsn/doc/
+	</ulink>
+  </para>
+</legalnotice>