Expand on how Git '[submodules] make managing dotfile dependencies so much easier'

[erikhuizinga]

The current text on Git submodules claims that '[they] make managing dotfile dependencies so much easier':

dotfiles.github.com/tips.md

Lines 16 to 18 in f0e70b7

	Consider using [Git submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules) as you
	start to add 3<sup>rd</sup> party frameworks, scripts, and plugins. Submodules make
	managing dotfile dependencies so much easier.

However, no further explanation is given why this may be the case and how this may be achieved over alternative methods. Consider expanding the section by adding or linking to such information.