Merge #21481: doc: Tell howto install clang-format on Debian/Ubuntu

ea76f4ac7d Doc: Tell howto install clang-format on Debian/Ubuntu (wodry)

Pull request description:

  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.

ACKs for top commit:
  hebasto:
    ACK ea76f4ac7d, every system upgrade via clean installation I do the same.

Tree-SHA512: 75c28540e8815cb41f4cf92784b6349978988b679e4deef9ae77ede951f93516ca13ec7b313ab72865b01273e115b49ed2b67cdcd68015af1b643a6186b190dd

contrib/devtools/README.md

@@ -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.