Clean up the README
Clean up the README

Shortened long lines; Markdown attaches no significance to single line
breaks.
Replaced various bold text headings with proper headings and
subheadings.

Replaced HTML tags:
  - replaced `<i>` with single asterisks;
    - replaced `<b>` (which should probably be `<strong>` anyway) with
      double asterisks;
        - replaced inline `<code>` tags with backticks

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