INSTALL.DPDK.md: Provide a little a more consistency to documentation.

INSTALL.DPDK.md: Provide a little a more consistency to documentation.

A few users (based on the reports in discuss@openvswitch.org) have been
literally following the instructions in INSTALL.DPDK.md and mixing up
pre-installed utilities and daemons with freshly compiled utilities
because the current documentation does not consistently call out
using utilities from the compiled sources.

This commit updates DPDK documentation to ask users to do a 'make install'
and then use the utilities and daemons directly from Linux PATH.
It also adds github markup where applicable.

Reported-by: Arkajit Ghosh <arkajit.ghosh@tcs.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>