[doc] improve the README file a bit

README

 This CubicWeb component provides a public registration feature (users
 can register and create an account without the need for admin
 intervention).
+
+To activate the user registration feature in your CubicWeb
+application, just add this cube in your instance ; either depend on it
+in your cube's __pkginfo__ or run the ``add_cude`` command in a
+CubicWeb shell::
+
+  $ cubicwebctl shell youapp
+  entering the migration python shell
+  just type migration commands or arbitrary python code and type ENTER to execute it
+  type "exit" or Ctrl-D to quit the shell and resume operation
+  >>> add_cube('registration')
+  >>> ^D
+
+
+Then add or modify the ``registration-cypher-seed`` configuration
+option in your application's ``all-in-one.conf`` file:
+
+  [REGISTRATION]
+
+  # seed used to cypher registration data in confirmation email link
+  registration-cypher-seed=<change me>
+
+