Update readme.md

Just fixing some weird opensource stuff.
Fix 'also bought' product names and undefined margin var.
Call wpsc_populate_also_bought_list() via 'wpsc_submit_checkout' filter.
Fix SQL IN error in wpsc_populate_also_bought_list().
Tidy up wpsc_also_bought() code and reference correct 'no image' image.
Fun fact: Walk Like an Egyptian was the Number One hit song the year I was born.  For bonus points, how old does that make me?