cli: update README of bash completion
cli: update README of bash completion

Add the description of how to customize bash_completion.

Dipsingh/gobgp,ramrunner/gobgp,a16/gobgp,ishidawataru/gobgp,tamihiro/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,ishidawataru/gobgp,h-naoto/gobgp,osrg/gobgp,tamihiro/gobgp,tamihiro/gobgp,YujiOshima/gobgp,Dipsingh/gobgp,fujita/gobgp,osrg/gobgp,YujiOshima/gobgp,YujiOshima/gobgp,tamihiro/gobgp,a16/gobgp,h-naoto/gobgp,fujita/gobgp,ishidawataru/gobgp,a16/gobgp,h-naoto/gobgp,a16/gobgp,fujita/gobgp,ramrunner/gobgp,a16/gobgp,j-kato/gobgp,ramrunner/gobgp,Dipsingh/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp