Add help2man dependency in README

README.md

 
 ### Manual
 
+You need the `help2man` package, required for install.
+
 `make && make install`
 
 **Optional:** If you want to use udev rules instead of suid to manage sysfs permissions, you may skip the `make install` step and instead add something like the following to `/etc/udev/rules.d/90-backlight.rules` after copying your binaries: