[docs] Mention `$XDG_DATA_HOME` as another possible save location

[pringshia]

Right now the docs only mention the old home dir save location:

tmux-resurrect/docs/save_dir.md

Line 3 in cff343c

By default Tmux environment is saved to a file in `~/.tmux/resurrect` dir.

Might be nice to also mention the save dir if the XDG spec is being used:

tmux-resurrect/scripts/helpers.sh

Line 4 in cff343c

default_resurrect_dir="${XDG_DATA_HOME:-$HOME/.local/share}"/tmux/resurrect