Guildhall installed in a different dir

[cordawyn]

When guildhall is installed in a different dir than guile/guild, all "extra" commands like "update" and so on are not available.

I suggest adding this info to README and/or docs/installation.texi:

If guildhall was installed into a custom location, which is different from the installation directory of guile, consider setting GUILE_FLAGS as follows:

export GUILE_FLAGS="-L $HOME/.local/share/guile/site/2.0"

(if you installed guildhall to $HOME/.local).

Additionally, if you installed the documentation by issuing "make install-info", adjust your INFOPATH too:

export INFOPATH="$HOME/.local/share/info"