Document possible gotcha related to /homeless-shelter

[collares]

By default, Nix builders set HOME to /homeless-shelter, under the expectation that creating it or writing to it will fail. Using nix-user-chroot, this can succeed. In my case, I was building Emacs and it wrote some files to /homeless-shelter instead of $out, causing them not to be included in the built package. In this particular case, Emacs' use of the home directory is probably an upstream bug caused by recent changes in emacs, but having a writable /homeless-shelter during build might expose bugs in other packages too, so perhaps documenting this possible problem is valuable.