Python 3.8 fixes.

README.rst

@@ -30,6 +30,17 @@ Usage::
   $ cd modoboa-installer
   $ sudo ./run.py <your domain>
 
+.. note::
+
+   On some systems (Ubuntu 20.04 for example), you might encounter the
+   following issue::
+
+     /usr/bin/env: 'python': No such file or directory
+
+   If so, try to run the command like this::
+
+     $ sudo python3 run.py <your domain>
+
 A configuration file will be automatically generated the first time
 you run the installer, please don't copy the
 ``installer.cfg.template`` file manually.