Mention `PackageCompiler.jl`

Julia compilation times may seem like a big disadvantage of the language to the students.

It may be good to at least mention `PackageCompiler.jl` which alleviates the issue. I don't have experience with the package, but it looks promising. Maybe mention it in [lecture 03](https://github.com/JuliaTeachingCTU/Julia-for-Optimization-and-Learning/blob/master/docs/src/lecture_03/otherpackages.md)?