This file is indexed.

/etc/sudoers.d/README is in sudo 1.8.3p1-1ubuntu3.7.

This file is owned by root:root, with mode 0o440.

The actual contents of the file can be viewed below.

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#
# As of Debian version 1.7.2p1-1, the default /etc/sudoers file created on
# installation of the package now includes the directive:
# 
# 	#includedir /etc/sudoers.d
# 
# This will cause sudo to read and parse any files in the /etc/sudoers.d 
# directory that do not end in '~' or contain a '.' character.
# 
# Note that there must be at least one file in the sudoers.d directory (this
# one will do), and all files in this directory should be mode 0440.
# 
# Note also, that because sudoers contents can vary widely, no attempt is 
# made to add this directive to existing sudoers files on upgrade.  Feel free
# to add the above directive to the end of your /etc/sudoers file to enable 
# this functionality for existing installations if you wish!
#