[README] Clarify custom installation instructions

[odow]

@ksun46 did you make sure these environment variables were set before calling using Ipopt?

If you previously ran using Ipopt, you will need to open a new Julia session for the changes to take effect.

x-ref: #247 (comment)

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Merging #248 (460f884) into master (79c1c52) will not change coverage.
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[ksun46]

@odow Yes I think so. I saw the very important note in the readme.

Just to confirm, we do not need a customized Ipopt to use HSL linear solvers. For example, every time when I start a new project, I will activate the project, and I would like to add the latest Ipopt and other packages into the toml files by doing Pkg.add("Ipopt"). I DONT need to set environment variables (to point to the customized Ipopt) for every project and Pkg.build("Ipopt"), right? I just need to make sure libhsl.dylib can be found somewhere in the load path (even before I start Julia)? Thanks.

[odow]

Just to confirm, we do not need a customized Ipopt to use HSL linear solvers

Correct.