adding docs about knife path config variables

plugins/knife/_knife

 #compdef knife
 
+# You can override the path to knife.rb and your cookbooks by setting
+# KNIFE_CONF_PATH=/path/to/my/.chef/knife.rb
+# KNIFE_COOKBOOK_PATH=/path/to/my/chef/cookbooks
+# Read around where these are used for more detail.
+
 # These flags should be available everywhere according to man knife
 knife_general_flags=( --help --server-url --key --config --editor --format --log_level --logfile --no-editor --user --print-after --version --yes )