message: Doc: Tell howto install clang-format on Debian/Ubuntu
  Because only macOS wasy mentioned, I was unsure if this would be a macOS specific tool. I guess Linux is more used than Mac, so Linux guide should be there, too.
diff: @@ -7,7 +7,8 @@ clang-format-diff.py
  A script to format unified git diffs according to [.clang-format](../../src/.clang-format).
  -Requires `clang-format`, installed e.g. via `brew install clang-format` on macOS.
  +Requires `clang-format`, installed e.g. via `brew install clang-format` on macOS,
  +or `sudo apt install clang-format` on Debian/Ubuntu.
  For instance, to format the last commit with 0 lines of context,
  the script should be called from the git root folder as follows.
  