README.rst: Change "wiki" to "documentation"
README.rst: Change "wiki" to "documentation"

Change-Id: I2acf959aa056779ad20d2583b184a151724986d5
Signed-off-by: Kyle Mestery <9cfbda34e625c7b0454bcb19a48d07c46f9097d2@mestery.com>

openstack/dragonflow,FrankDuan/df_code,openstack/dragonflow,FrankDuan/df_code,openstack/dragonflow,FrankDuan/df_code