Document GPL status, and make GPL-free option

This package doesn't make clear that the binary it makes is (maybe) GPL-licensed.

People may think that they can make proprietary Julia source code (true) and distribute it as such (true), and with Julia (unclear, "mere aggregation"?), and hide the source-code using your package. They can hide it yes, and many want that for their propriatary code, but may not realize that they then distribute under GPL (since Julia is GPLed), with a promise to provide the source code (i.e. make it free).

In addition:
https://copyleft.org/guide/comprehensive-gpl-guidech7.html#x10-490006
>GPL’s Implied Patent Grant

You can make a GPL-free Julia (a make option), as you know, but it's not the default, and unclear (unlikely) your package uses that (could it?), but it will be the default with Julia 1.9 (or rather LGPLed, not MIT):

https://github.com/JuliaLang/julia/pull/44247#issuecomment-1166336192

I see in the docs (unclear to me if filtering out SuiteSparse is enough, for Julia 1.8 and lower, but maybe it should be the default option):

>By default, all standard libraries are included in the sysimage. It is possible to only include those standard libraries that the project needs. This is done by passing the keyword argument filter_stdlibs=true to create_app. This causes the sysimage to be smaller, and possibly load faster.

Start with the create_app documentation, especially the filter_stdlibs=true behavior, and review the linked Julia licensing and SuiteSparse references. Clarify the package's current GPL status and determine the supported GPL-free build option; done means the documentation and option behavior are explicit and verifiable.

Written by the indexing model from the issue text.