Warning on using GUIX_PACKAGE_PATH

[Pjotr Prins]

@FOSDEM we concluded that GUIX_PACKAGE_PATH does not necessarily work
that wel. I added to my guix-notes the following:

+Note that, even though GUIX_PACKAGE_PATH can be a feasible way of
+adding and maintaining packages, it has two largish downsides: (1) it
+is removed from the main package tree and therefore not easily shared
+and integrated and (2) to remain compatible you need to juggle two git
+trees which may go out of synch.