Add comment warning about modifying landmines

build/landmines.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 This script runs every build as the first hook (See DEPS). If it detects that
 the build should be clobbered, it will delete the contents of the build
 directory.
 
 A landmine is tripped when a builder checks out a different revision, and the
 diff between the new landmines and the old ones is non-null. At this point, the
 build is clobbered.
+
+Before adding or changing a landmine consider the consequences of doing so.
+Doing so will wipe out every output directory on every Chrome developer's
+machine. This can be particularly problematic on Windows where the directory
+deletion may well fail (locked files, command prompt in the directory, etc.),
+and generated .sln and .vcxproj files will be deleted.
+
+This output directory deletion will be repated when going back and forth across
+the change that added the landmine, adding to the cost. There are usually less
+troublesome alternatives.
 """
 
 import difflib
 import errno
 import gyp_environment
 import logging
 import optparse
 import os
 import sys
 import subprocess
@@ skipping 105 matching lines @@
     proc = subprocess.Popen([sys.executable, s], stdout=subprocess.PIPE)
     output, _ = proc.communicate()
     landmines.extend([('%s\n' % l.strip()) for l in output.splitlines()])
   clobber_if_necessary(landmines, options.src_dir)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())