Scripts and tools & Docs: Used #!/usr/bin/env bash instead of obsolete #!/bin/bash, added linting for .sh files shebang and updated the Developer Notes.

contrib/qos/tc.sh

@@ -1,3 +1,5 @@
+#!/usr/bin/env bash
+#
 # Copyright (c) 2017 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.