Add comment warning about modifying landmines

Modifying landmines has greater consequences than is sometimes realized
for developers, especially on Windows. Add a comment to warn editors of
these consequences.

This was inspired by https://chromium-review.googlesource.com/c/544867/

R=dpranke@chromium.org
BUG=736939

Review-Url: https://codereview.chromium.org/2959743004
Cr-Original-Commit-Position: refs/heads/master@{#482639}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 46ac6b742672a82001cd3d8ef0c7525be72a9097

landmines.py

@@ -11,6 +11,16 @@ 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