Add the PGA's producers mark to relevant credits

Since now the missing big studios have come on board last month, the PGA producers mark will become much more common and important.

More info: http://variety.com/2013/film/news/dis...

I think it is critical for IMDb to adapt that mark in their credits as well and do so as soon as possible.

For one it's a very important attribute, esp with the flood of producers many movies have nowadays. It would be really good to see which one of the producers is actually doing the work and who got the credit only because s/he was a good negotiator.

And I think it would be a good idea to implement it asap if only for the fact that right now the number of movies/producers who have been awarded the mark already is still fairly small, so adding the existing credit attributes in hindsight should be fast. If you wait too long, the number of credits to update becomes bigger and bigger.

I assume you could use the standard attribute field to add the "p.g.a." manually right now but I think it should be a more standardized option.