add "how to write package::sub" in p 5 --> 6 guide

[lichtkind]

The problem

I'm not sure if belongs into https://docs.perl6.org/language/5to6-perlsyn, so I won't just edit the pages, but somewhere in the migration docs should be mentioned, that in order to do as in P5, you have to name your package a module and make the subs our, so that their can be called from outside. (its the wrong default IMO anyway) but i think that would help greatly.

[JJ]

OK, we'll try to take a look at that.

[lichtkind]

thanks