Missing proper documentation on how to cross compile with dub

[crazymonkyyy]

As far as I can tell https://wiki.dlang.org/Cross-compiling_with_LDC#Cross-compiling_with_dub is all the information that exist, so when you run into issues, you can only grasp at straws.

[Geod24]

CC @kinke

[kinke]

As far as dub itself is concerned, its only job is to forward the --arch target triple to all LDC invocations (as -mtriple) [besides figuring correct file extensions etc. for the target], so most potential issues arise from an inadequate LDC setup. - Slightly extending the dub cmdline help or whatever can be done by anyone, just like updating the Wiki pages.