Update read me for sizes more than 4MiB

[balaji-ch]

Hi ,

I faced the same issue as stated in
#25
and the solution given in the same link

insmod u-dma-buf.ko dma_mask_bit=64 udmabuf0=8388608

solves the issue. Please update the usage of dma_mask_bit in the readme .

[KAWAZOME Ichiro]

Thank you for the issue.