doc: show how to include code in guides

doc: show how to include code in guides

Currently the documentation describes how to add code snippets to the
docs using code blocks. This can be problematic as the code snippets
in the docs may fall out of sync with the actual code it is referencing
within DPDK. This patch adds instructions to the contribution guide
about how to include code in the docs using literalinclude which will
dynamically get the code from source when the docs are generated. This
will help to ensure that the code within the docs is up to date and not
out of sync with the actual code.

Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>