Added documentation to manually enable required extensions

docs/installation/external_dependencies.rst

@@ -43,6 +43,15 @@ you should now be able to open a postgresql shell:
 
     sudo -u funkwhale -H psql
 
+Unless you give a superuser access to the database user, you should also
+enable some extensions on your database server, as those are required
+for funkwhale to work properly:
+
+.. code-block:: shell
+
+    sudo -u postgres psql -c 'CREATE EXTENSION "unaccent";''
+
+
 Cache setup (Redis)
 -------------------