Include information about building system image

[AustinPrivett]

I have been using Julia for a couple of months now and did not come across http://www.stochasticlifestyle.com/7-julia-gotchas-handle/ until recently.

If building the system image still does what is mentioned at that website (i.e., allow all compiler optimizations unlike the pre-built binaries), I think it is helpful to point that out early on to give new users better performance.

[ViralBShah]

I feel like this is something that belongs somewhere in the Julia docs - but not directly on the downloads page. I wonder what others think.

[ararslan]

I agree. To me this is more "advanced user" territory that 99% of users shouldn't need to care about. @AustinPrivett, would you be willing to submit a PR to Julia to add this information to the documentation?

[fredrikekre]

See also https://github.com/JuliaLang/julia/issues/24646

[KristofferC]

For Julia 0.7 this is not accurate anymore (pre built binaries are built for multiple targets and jit compiled code is compiled for native target).

[AustinPrivett]

In light of the modifications to Julia 0.7 mentioned by KristofferC, I'm closing this and not worrying about adding this information to the documentation.