README: Adjust heading style (jsc#PED-5021)
    
    * Underscore all headings as a preparation for Markdown conversion.
    * Use title-style capitalization for the document name and
      sentence-style capitalization for section headings, as recommended in
      the current SUSE Documentation Style Guide.
    * Strip the table of contents. The document is short and easy to
      navigate just by scrolling through it.