ANSWER:

Even better, you configure Vim to watch for changes in your .vimrc and automatically reload the config.

augroup myvimrc
    au!
    au BufWritePost .vimrc,_vimrc,vimrc,.gvimrc,_gvimrc,gvimrc so $MYVIMRC | if has('gui_running') | so $MYGVIMRC | endif
augroup END

Source: this answer on SO

Note: this particular method watches for the many variations of Vim config filenames so that it’s compatible with GUI Vim, Windows Vim, etc.

by Andrei from http://superuser.com/questions/417997