Added note to README about where to go for upgrade instructions.

README

@@ -6,6 +6,10 @@ phase. To install, please follow the instructions here:
 
   http://www.mahara.org/wiki/En/Sysadmin_Guide/Installing_Mahara
 
+If you already have a Mahara you wish to upgrade, follow the instructions here:
+
+  http://www.mahara.org/wiki/En/Sysadmin_Guide/Upgrading_Mahara
+
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