Add `vim-javascript` to README's Vim plugin list
Add a note on changes required for forks

Make it clear that if this project is forked, the developer should
modify the variable values in `bin/dotfiles` that contain paths to my
dotfiles. These variables are found at the top of the file and are easy
to change.
Suggest installing iTerm2 and the Solarized theme

The custom prompt mimicks Solarized's colours but for a complete
experience it's best to install iTerm2 and use the Solarized Dark theme
to provide the terminal colour scheme.