Change README.md: recommend yes for all dialog on first-time install

I added a recommendation that first-time users choose yes for all
questions on the first installation. This is because, otherwhise, you
will most likely run in to issues without all of the setup the install
script provides.
Change README.md: add A Note to Users section

I added a section title "A Note to Users" describing what to do in the
event that the vim-airline characters do not show up correctly.