updated ``Installation Procedure`` section to add several information.

- a quick description of each part just to know what to install
  - state that modules are addons for the server (which might not be clear to reader)
  - imply that tryton is a client and other client exists.
- removed the ``$`` because we want people to be able to cut and paste
- remaining typo: modules where incorrectly tryton_module instead of trytond_module...
- don't force virtual env, but recommend it.
- where to get list of modules.
- separate clearly each install commands.
Could you remove the *legacy* word?