Document why some stdlib files are not copied over

Author: eregon

tool/import-mri-files.sh

@@ -19,14 +19,19 @@ topdir=$(cd ../ruby && pwd -P)
 rm -r lib/mri
 cp -r ../ruby/lib lib/mri
 rm lib/mri/racc/rdoc/grammar.en.rdoc
+# We have our own version under lib/truffle
 rm lib/mri/securerandom.rb
 rm lib/mri/timeout.rb
 rm lib/mri/weakref.rb
-rm lib/mri/profile.rb
+# Uses TracePoint.new(:call), and we have a more efficient --cpusampler
 rm lib/mri/profiler.rb
+# Uses RubyVM
 rm lib/mri/debug.rb
+rm lib/mri/profile.rb
+# NPE in for
 rm lib/mri/shell.rb
 rm -r lib/mri/shell
+# Files not actually installed in MRI
 find lib/mri -name '*.gemspec' -delete
 find lib/mri -name '.document' -delete