Document the agentless mode

[aylei]

Now that #31 introduced the agentless mode, we should document this feature accordingly.

[aylei]

@tkanng Are you interested in documenting this as the feature developer (either Chinese/English version)?

I can do this if you don't have spare time recently.

[tkanng]

It's my pleasure. I'll do it this week. :)

[aylei]

close via #33 Thanks @tkanng