settings: add auto-generated warning msg for editing the file manually

Hopefully, refraining users from modifying the file unless they are
certain about the potential consequences.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/qt/test/optiontests.cpp

@@ -61,7 +61,11 @@ void OptionTests::migrateSettings()
     QVERIFY(!settings.contains("addrSeparateProxyTor"));
 
     std::ifstream file(gArgs.GetDataDirNet() / "settings.json");
+    std::string default_warning = strprintf("This file is automatically generated and updated by %s. Please do not edit this file while the node "
+                                            "is running, as any changes might be ignored or overwritten.",
+                                            PACKAGE_NAME);
     QCOMPARE(std::string(std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>()).c_str(), "{\n"
+        "    \"_warning_\": \""+ default_warning+"\",\n"
         "    \"dbcache\": \"600\",\n"
         "    \"listen\": false,\n"
         "    \"onion\": \"onion:234\",\n"