v2.5.1 updates to changelog (Xilinx#1015)

Update README.md and changelog for v2.5 (Xilinx#959)

Add register map text to the overlay tutorial (Xilinx#821)

Minor formatting fixes (Xilinx#733)

Update getting started video links

Update pynq_sd_card.rst

Looks to me the path was incorrect

v2.1 doc updates (Xilinx#581)

links now to point to v2.0 image zipfile

Merge image_2017_01 into master

fix temp file for minor doc change