Add backends doc

database_backends.txt

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+======== PostgreSQL, MySQL and SQLite databases
+
+Bacula director supports different databases backends, if you want to switch
+away from the default PostgreSQL one you need to change the "libbaccats" (the
+catalogue library) symlink to the real library.
+
+To change to a different backend, issue the following command:
+
+# alternatives --config libbaccats.so
+
+There are 3 programs which provide 'libbaccats.so'.
+
+  Selection    Command
+-----------------------------------------------
+   1           /usr/lib64/libbaccats-mysql.so
+   2           /usr/lib64/libbaccats-sqlite3.so
+*+ 3           /usr/lib64/libbaccats-postgresql.so
+
+Enter to keep the current selection[+], or type selection number: 1
+
+There is NO need to edit any part in the Bacula Director configuration; for the
+purposes of the database creation steps, the bacula-dir.conf configuration file
+can be left at their default values.
+
+======== Switiching between PostgreSQL, MySQL and SQLite backends
+
+To switch the configured backend to another one, follow the above procedure.
+Again, there's no need to edit the Bacula Director configuration file; the
+catalog resource can be left as is.
+
+Importing and exporting data between the various database formats is up to the
+user. If the database will be re-initialized from scratch, follow the quick
+start guides mentioned above.