README: add a section about API documentation
README: add a section about API documentation

Doxygen and python documentation are briefly mentioned in the BINDINGS
section but there's no word about the core C API and automatically
generated man pages.

Add a separate section to the README file describing the process of
generating html doxygen output, python help strings and man pages for
gpio-tools.

Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

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