Add how to precompile into readme

[lstark-uibk]

in my case i loaded the program with vsc github and everything was downloaded.
but then I failded to precompile everything

So just state: before first time running:
run in julia
using Pkg
Pkg.precompile()
if this is not working (as was the case with me) run Pkg.update()
Pkg.activate .

Or another way round (I donot really know the details)