some minimalist docs for Xapian

Author: Richard Jones

doc/installation.txt

@@ -43,6 +43,22 @@ have to install the win32all package separately (get it from
 http://starship.python.net/crew/mhammond/win32/).
 
 
+Optional Components
+===================
+
+You may optionall install and use:
+
+An RDBMS
+  Sqlite, MySQL and Postgresql are all supported by Roundup and will be
+  used if available. One of these is recommended if you are anticipating a
+  large user base (see `choosing your backend`_ below).
+Xapian full-text indexer
+  The Xapian_ full-text indexer is also supported and will be used by
+  default if it is available. This is strongly recommended if you are
+  anticipating a large number of issues (> 1000).
+
+.. _Xapian: http://www.xapian.org/
+
 Getting Roundup
 ===============