Document using show instead of serve

[spanezz]

ssite serve will read settings.py or .staticsite.py files and execute them. This means that if it is used to preview random git repositories, a repository with such a file can get it executed. This can be both a security issue and an annoyance, as in for example previewing a django project.

Document that ssite show does not have this issue and should be used by default. ssite serve is still useful to serve a site that the user has control of.