Add a specific "Update" section to the README
Add a specific "Update" section to the README

Thanks to rcm and the `post-up` hook file, the dotfiles do a great job
of keeping things up to date.

This change adds a bit of detail and specificity to the README around
updating, specifically clarifying the need to run `rcup` after pulling
to link new files and install new vim plugins.

bschmoker/dotfiles,YepengFan/dotfiles,YepengFan/dotfiles,bigbam505/dotfiles-work,koerbcm/dotfiles,turino/dotfiles,tchiarato/dotfiles,moofish32/dotfiles,moofish32/dotfiles,astoellis/dotfiles,ecnalyr/dotfiles,koerbcm/dotfiles,ren/dotfiles,bigbam505/dotfiles-work,koerbcm/dotfiles,koerbcm/dotfiles,koerbcm/dotfiles,ren/dotfiles,tchiarato/dotfiles,bschmoker/dotfiles,astoellis/dotfiles,toobulkeh/dotfiles