Update README.txt

Move from http://drupal.org/node/1097092#comment-4231388 reported by chriscjcj:

Thank you very much for your assistance.

I did as you said and was able to successfully restore the Left sidebar, (now called sidebar_first.) I don't think I would've figured that one out on my own.

I re-read the README.txt to see if this was mentioned, and I didn't see it in there. The other thing I struggled with on this upgrade was making sure Zen was installed; at first I didn't realize this was necessary. Perhaps these necessary steps should be mentioned in the README.txt.

Thanks again and best wishes,
-Chris Johnson