/usr/share/doc/fonts-texgyre/README-TeX-Gyre-Bonum-Math.txt is in fonts-texgyre 20150923-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/fonts-texgyre/../../texmf/doc/fonts/tex-gyre-math/README-TeX-Gyre-Bonum-Math.txt