Add more information about stumbling blocks when installing PMACS data

[tsalo]

Just a few notes from my attempts today:

1. You might need to add module load git to your PMACS .bashrc.
2. The documentation says after which, you will see some [INFO ] messages that look scary, but are harmless and expected., but I got some scary [WARNING] messages as well, including [WARNING] remote HEAD refers to nonexistent ref, unable to checkout.
3. I need to put in my PMACS password twice every time I datalad get anything in the dataset.