Re-write the tutorial

Thanks to Rebecca Meritz (@rmeritz) for feedback on rcm(7), I have
restructured it and re-written the quick start section. Much of the
details about the sync algorithm have moved into rcup(1).

The new rcm(7) covers a quick start for those with existing directories,
including caveats for `install` scripts, dotted filenames, and
non-~/.dotfiles directory names; a quick start for those without
anything; and motivating sections for "advanced" features like tags,
host-specific files, and multiple directories.