Remove postinstall and instead print a text explaining what to do and pointing to the documentation

[alexAubin]

This proposes to remove the postinstall being proposed / triggered at the end of the install script.
Motivations behind this is :

• simplify the command line. Currently we have to run bash <(wget the_url) instead of bash the_url | curl because bash needs access to stdin only for the postinstall that might be run ;
• imho there's little gain in triggering this from the install script rather than asking the user to do it from the web admin or the cli (so that the admin discovers one or the other rather than the install script masking it ?) ;
• if the postinstall fails during the install script, it's not easy to debug :|, c.f. Not information on failed post-install while using install.yunohost.org issues#1166
• we should point at the documentation more often and that's a good occasion to do so ?

[zamentur]

LGTM