Update README.md installation instructions.
Update README.md installation instructions.

`run '~/.tmux/plugins/tpm/tpm'` should be run in the background, so the `-b` flag was added.
joshuarubin/dotfiles,tmux-plugins/tpm,bruno-/tpm,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles