Ignore all files under wiki/ except README

Signed-off-by: James Ketrenos <james.p.ketrenos@intel.com>
Fix permission conflict by wiki/ being in GIT when running update

crosswalk-project.git contains wiki/ which needs to be owned by
wwwrun:www in order for the Apache process to be able to write to it
(eg., gfm.php and regen.php) However the update process requires that
directory to be writeable by drush:users

Signed-off-by: James Ketrenos <james.p.ketrenos@intel.com>