Added changelog entries for 5.1 and 6.0

Doc/changelog.docu

-/** \page om_changelog Changelog
+/* \page om_changelog Changelog
 
 \htmlonly
 
@@ -7,6 +7,20 @@
 <!-- --------------------------------------------------------------------- -->
 
 
+<tr valign=top><td><b>6.0</b> (?/?/?)</td><td>
+
+
+</tr>
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+<tr valign=top><td><b>5.1</b> (?/?/?)</td><td>
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+
+</tr>
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+
 <tr valign=top><td><b>5.0</b> (2015/11/18)</td><td>
 
 <b>General</b>