please document the pkg-ng repo problem

[Daniel Herring]

Hi,

pkg-ng has been broken for a year now (the public repo is nearly useless), 
and I see no indication when the repo will be rebuilt.

It would have saved me several hours if the Handbook advised users to not 
run "pkg2ng" until the repo is actually ready.

http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/pkgng-intro.html

https://wiki.freebsd.org/pkgng#Availability_of_binary_pkgs_for_Download

Thanks,
Daniel

P.S.  IMO, this is a rather significant problem with the 10-beta.