Update author_letter.R

tried using 
newimg@grey[rows, cols] <- randomnoise
newimg@grey <- (1-q) * img@grey + q * randomnoise
but got "non-conformable arrays" error
newimg@grey[rows,cols] <- (1-q) * img@grey[rows,cols] + q * randomnoise
is required
Merge pull request #1 from tkumark/patch-1

Update author_letter.R