doc: Document that GNU tools are required for linters

Author: MarcoFalke

test/lint/README.md

@@ -7,6 +7,8 @@ Check for missing documentation of command line options.
 commit-script-check.sh
 ======================
 Verification of [scripted diffs](/doc/developer-notes.md#scripted-diffs).
+Scripted diffs are only assumed to run on the latest LTS release of Ubuntu. Running them on other operating systems
+might require installing GNU tools, such as GNU sed.
 
 git-subtree-check.sh
 ====================