fix(supann-ext) the supann-ext plugin missed the register command

Signed-off-by: Benoit Mortier <benoit.mortier@fusiondirectory.org>

source/fusiondirectory/plugins/supann-ext/installation.rst

@@ -35,4 +35,11 @@ RHEL
 
 .. code-block:: bash
 
-    fusiondirectory-schema-manager --insert-schema /etc/ldap/schema/fusiondirectory/supann-ext-fd.schema
\ No newline at end of file
+    fusiondirectory-schema-manager --insert-schema /etc/ldap/schema/fusiondirectory/supann-ext-fd.schema
+
+Register the plugin
+-------------------
+
+.. code-block:: bash
+
+   fusiondirectory-plugins-manager --register-plugin /etc/fusiondirectory/yaml/supann-ext/description.yaml