expand comment

.gitignore

@@ -1,6 +1,10 @@
-# This file should only ignore things that are generated during a build,
-# generated by common IDEs, and optional files controlled by the user
-# that affect the build (such as config.toml).
+# This file should only ignore things that are generated during a `x.py` build,
+# generated by common IDEs, and optional files controlled by the user that
+# affect the build (such as config.toml).
+# In particular, things like `mir_dump` should not be listed here; they are only
+# created during manual debugging and many people like to clean up instead of
+# having git ignore such leftovers. You can use `.git/info/exclude` to
+# configure your local ignore list.
 # FIXME: This needs cleanup.
 *~
 .#*
@@ -52,3 +56,4 @@ config.stamp
 Session.vim
 .cargo
 no_llvm_build
+# Before adding new lines, see the comment at the top.