README: add info about building from release tarballs
README: add info about building from release tarballs

There's no need to use autogen.sh if building from release tarballs as
the configure script is already provided. Only git builds need
autoreconf.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod