Add a note that landmines should not be used in general.

build/get_landmines.py

Index: build/get_landmines.py
diff --git a/build/get_landmines.py b/build/get_landmines.py
index d7c98a5fb13e334bf5761937c4d64bbe881370f7..8f5b87895dafa561fee34716e20fa71d3a33321e 100755
--- a/build/get_landmines.py
+++ b/build/get_landmines.py
@@ -24,6 +24,12 @@ def print_landmines():
   """
   ALL LANDMINES ARE EMITTED FROM HERE.
   """
+  # DO NOT add landmines as part of a regular CL. Landmines are a last-effort
+  # bandaid fix if a CL that got landed has a build dependency bug and all bots
+  # need to be cleaned up. If you're writing a new CL that causes build
+  # dependency problems, fix the dependency problems instead of adding a
+  # landmine.
+
   if (distributor() == 'goma' and platform() == 'win32' and
       builder() == 'ninja'):
     print 'Need to clobber winja goma due to backend cwd cache fix.'