README: Document --with-system-libblkmaker

README

@@ -75,6 +75,8 @@ BFGMiner specific configuration options:
 	--disable-x6500         Compile support for X6500 (default enabled)
 	--disable-ztex          Compile support for ZTEX (default if libusb)
 	--enable-scrypt         Compile support for scrypt mining (default disabled)
+	--with-system-libblkmaker  Use system libblkmaker rather than bundled one
+	                           (default disabled)
 	--without-curses        Compile support for curses TUI (default enabled)
 	--without-libudev       Autodetect FPGAs using libudev (default enabled)