HACKING: Change format to MarkDown

HACKING: Change format to MarkDown

* HACKING: Converted initially from LaTeX to markdown with:

  'pandoc -f latex -t markdown HACKING.tex'

  Then tweaked by hand to add a header with some suitable variables for the
  pandoc LaTeX template to make better use of the page space, and to add
  newpages so the title page is standalone as in the previous version.

  Also went through and reflowed paragraphs best as I could to make them
  match the previous document, to make it a little easier to verify nothing
  had been changed other than the markup, via diff.

  PDF can be generated with: 'pandoc  -o HACKING.pdf HACKING.md'

(cherry picked from commit 67c3d75f5324b610352998c670f5f0cc4ba0ff2a)