docs: Explain the subleties of Sandstorm app auto-update

[paulproteus]

References:

• subject="What happens if" on sandstorm-dev: https://groups.google.com/forum/#!topic/sandstorm-dev/yK9PeAEGyeA

Probably it could go within the "using" section, in a new file called "Automatic updates", which mostly talks about app auto-updates but maybe maybe says Sandstorm itself also auto-updates.