add BRAINSCORE_HOME to docs

From @ShreyaKapoor18 in https://github.com/brain-score/vision/issues/533#issuecomment-1959217245:

> One thing I'd like to mention is that maybe it is helpful to add the following things to the documentation
> `export BRAINSCORE_HOME="/path/to/custom/brainscore/dir"`
> since the brain score results were being stored in the $HOME (with limited memory) directory for my cluster storage by default.