add dependency manager link

[knathanieltucker]

in the readme

I also googled "dependency manager." It is about what I expected—but I don't think a lot about environments or dependencies. I'm aware of them, but, again, I really don't understand them. Potentially something to address in further detail in
another series.

@PaulHarnagel

[PaulHarnagel]

P1 updated w/link:
https://medium.com/prodsters/what-are-dependency-managers-26d7d907deb8