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.

Author: wodry

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.