prerequisites: Add more reference links for git and AsciiDoc
This commit adds an internal link to @bcotton's git for docs writer
page, and also adds a link to the AsciiDoc quick reference syntax. It is
reasonable to assume someone does not have prior experience with
AsciiDoc since it is an uncommon markup language. So, this link provides
a practical reference to learn the syntax. (I refer to this page almost
every time I write Fedora Docs.)
Signed-off-by: Justin W. Flory <git@jwf.io>