/etc/fonts/conf.d/README is in fontconfig-config 2.8.0-3ubuntu9.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Each file in this directory is a fontconfig configuration file.  Fontconfig
scans this directory, loading all files of the form [0-9][0-9]*.conf.
These files are normally installed in ../conf.avail and then symlinked here,
allowing them to be easily installed and then enabled/disabled by adjusting
the symlinks.
The files are loaded in numeric order, the structure of the configuration
has led to the following conventions in usage:
 Files begining with:	Contain:
 
 00 through 09		Font directories
 10 through 19		system rendering defaults (AA, etc)
 20 through 29		font rendering options
 30 through 39		family substitution
 40 through 49		generic identification, map family->generic
 50 through 59		alternate config file loading
 60 through 69		generic aliases, map generic->family
 70 through 79		select font (adjust which fonts are available)
 80 through 89		match target="scan" (modify scanned patterns)
 90 through 99		font synthesis
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