I'm unsure what you've seen or read so far, but I found a few sites that might have some information that you don't so I'll post them...although you've probably read it all.
http://www.countingdown.com/movies/290820/news
http://www.killermovies.com/e/exorcistthebeginning/
http://www.pop.com/movies/290820
Most of it is repeating information though.
And the supposed storyline:
"Set in the late 1940s, a prequel which reveals the life-changing event that took place in Father Merrin's existence, when he was sent as a young missionary priest on a journey to post-WWII Africa. His faith had been tested and shaken by his recent experiences in the hellish war, and thus his spirit was vulnerable. Father Merrin's journey leads him to his first encounter with demonic forces, as he comes in contact with an age-old evil, while on an archeological dig in British East Africa."
Source:
http://movies.yahoo.com/shop?d=hv&id...f=info&intl=us