doc: block diagram of HackRF

Source: http://2013.hackitoergosum.org/presentations/Day2-04.HackRF%20A%20Low%20Cost%20Software%20Defined%20Radio%20Platform%20by%20Benjamin%20Vernoux.pdf
Merge pull request #98 from mikeryan/blockdiagram-images

doc: block diagram of HackRF