Rewrite readme.

Changes of interest:

* Remove the "last update to m-file" comment. Git tracks who changed what and
  when, so there is little value in tracking this information separately. To the
  contrary, having multiple sources of information for a single fact makes it
  hard to tell which one is the true and authoritative source, and which one is
  simply outdated and wrong.
* Change headline levels. It is nonsensical to have level 3 and level 4
  headlines, but no level 1 or level 2 headlines. The purpose of HTML (and
  markdown) is to state semantically correct information. (If headlines are too
  large, do not demote the headline. That's what CSS is for.)
* Limit lines to a maximum width of 80 characters. Short lines are easier to
  diff and read. For many text editors, they're also easier to edit.
* Change tabs to spaces. Strip trailing whitespace.
* Change the first headline from "Introduction" to "VertConst".
* Demark headlines with "===" and "---", not "###". Leading hashes are hard to
  comprehend in a plain text editor, especially as the number of hashes
  increases.