Add doc directory and files

doc/swarm_network_registers.md is a Markdown file that describes the
network related registers of the SWARM correlator design (aka sma_corr).

doc/swarm_network_registers.pdf is a PDF rendering of the corresponding
Markdown file.

doc/Makefile uses "pandoc" to generate the PDF file from the Markdown
file.
Add network/sync_count to docs