Add contributors.txt file

As per the discussion in SMF Development, Arantor has suggested the use of a contributors text file to replace the credits. This file list everybody (I hope) that's either submitted code to SMF or been on the SMF Team. Also includes a small special thanks section and credits to third party software and graphics.

P.S: This is just a base for others to work off, most noticeable missing is SCEditor.

Signed-off-by: Acans <acans@simplemachines.org>