- correct CHANGELOG title levels: since readme uses "----" for the
  first level and "~~~~" for the second level, "====" was interpreted as
  the third title level and "Pygolang change history" became
  sub-sub-section of "Additional packages and utilities" with section
  for each version conversely becoming first level. It was not very
  noticeable until 0c5f9d06 (readme: Push "Additional packages and
  utilities" into its own section) started to use "~~~~".
  -> Rework CHANGELOG titling to align with the one in README.

- fix minor markup bits in README.