Updated credits in handbook

[Jack]

I just pushed my git commit for the credits chapter of the handbook.  I  
reorganized it a bit to follow the changes Łukasz made to the credits  
in the "about" dialog.

I would appreciate all contributors checking this to see I have not  
inadvertently edited anyone out.

Also, there are some contributors in my list who are not on the "about"  
list.  I assume they have all been inactive for a while, but someone  
with a longer memory than mine might confirm whether or not they should  
be included there also.

Jack