Added a conventions guide to the top

As I'm writing the transactions section, I discovered I didn't want to keep mentioning that I'd elided bits from long strings and output, so I'm adding a conventions section to the top of the file.  This follows the style of most technical books of describing in the beginning some special features which make the text more readable.

Note: the preview on github is broken; this is probably a result of some of the divs Saïvann added to make the page work with the new bitcoin.org layout.  I opened issue#10 about it.