Add explanation about npm packages

[f-f]

Explain why my proposal here doesn't work:
https://discourse.purescript.org/t/post-purescript-package-management-explained/615/7

This thread is also highly informative on the matter:
purescript/purescript#631 (comment)

[chexxor]

Nice explanation. I believe it's likely that this won't prevent/end future requests for spago to integrate with other build/dep management tools, but this at least gives a common starting point for those discussions. I think it's possible there's still some ways to support these other tools in a limited scope, so perhaps those future discussions will have proposals for them.

If you'd like to more absolutely block future requests for support for other dep tools, I suggest putting a more pointed sentence in there to state so.

[f-f]

@chexxor: integrating with other tools is hard and it's a lot of work, but it's an actual UX issue.
I would love myself to not have to use npm and yarn and lerna and whatever else. Setting up the JS+PS build for our monorepo at work was very painful.

So I wouldn't like to exclude this possibility, as someone might come up with a clever solution and PR it 😄