README should mention libharfbuzz-dev as a dependency

[lpanebr]

If libharfbuzz-dev is not installed you get a build error.

sudo make install                
c99 -I/usr/X11R6/include  `pkg-config --cflags fontconfig`  `pkg-config --cflags freetype2`  `pkg-config --cflags harfbuzz` -DVERSION=\"0.8.2\" -D_XOPEN_SOURCE=600  -O -c x.c
Package harfbuzz was not found in the pkg-config search path.
Perhaps you should add the directory containing `harfbuzz.pc'
to the PKG_CONFIG_PATH environment variable
No package 'harfbuzz' found
In file included from x.c:23:
hb.h:2:10: fatal error: hb.h: No such file or directory
    2 | #include <hb.h>
      |          ^~~~~~
compilation terminated.
make: *** [Makefile:22: x.o] Error 1