[Feature request] coexist with existing Julia instals

[LilithHafner]

A discussion on discourse brought to light that juliaup (sometimes) works with existing Julia installs, the readme currently recommends against this. It would be great if that cooperation could be formally supported.

[davidanthoff]

I think this is really a documentation task, we should just point out that one can put any Julia binary anywhere, and then use the juliaup link command to integrate that Julia version into the versions surfaced by Juliaup.

We probably also want to highlight that things won't work if a different julia binary is on the PATH...

[rawjeev]

Was going to try out juliaup and first thing that popped in my head is, what should I do with my current installations? Nice to know, that isn't an issue. And I think documentation should have how to uninstall juliaup, just in case. Also I am wondering what happens to all the Julia installations that juliaup is multiplexing with when we happen to remove juliaup!