doc: add af_packet PMD guide

doc: add af_packet PMD guide

As of commit 364e08f2bbc0, DPDK allows an application to send and
receive raw packets using an AF_PACKET and PACKET_MMAP, when using
Linux Kernel. This complements it by adding a simple guide with the
following information:
- An introduction, where a brief explanation of this driver is given,
  pointing out the dependency on PACKET_MMAP;
- Which options are supported at configuration time, while setting up an
  interface, and it's inherent limitations;
- What the prerequisites are;
- A command line example of how to set up a DPDK port using the
  af_packet driver.

Since there's a dependency in PACKET_MMAP, the guide also points to the
original Kernel documentation, so the reader can get more details.

Signed-off-by: Tiago Lam <tiago.lam@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>