doc(var) load default var indexes (#35)

* add desc for lua_kong_load_default_var_indexes

* fix doc link

* update desc for lua_kong_load_var_index

* Update README.md

Co-authored-by: Wangchong Zhou <fffonion@gmail.com>

Co-authored-by: Wangchong Zhou <fffonion@gmail.com>

README.md

@@ -54,7 +54,7 @@ Directives
 
 lua_kong_load_var_index
 -------------------------------------------
-**syntax:** *lua_kong_load_var_index $variable;*
+**syntax:** *lua_kong_load_var_index $variable | default;*
 
 **context:** *http*
 
@@ -73,6 +73,49 @@ Common variables defined by other modules that are already indexed:
 - `$http_user_agent`
 - `$host`
 
+Specially, use `lua_kong_load_var_index default` to
+index *commonly used variables* as follows:
+
+- `$args`
+- `$is_args`
+- `$bytes_sent`
+- `$content_type`
+- `$http_authorization`
+- `$http_connection`
+- `$http_host`
+- `$http_kong_debug`
+- `$http_proxy`
+- `$http_proxy_connection`
+- `$http_te`
+- `$http_upgrade`
+- `$http_x_forwarded_for`
+- `$http_x_forwarded_host`
+- `$http_x_forwarded_path`
+- `$http_x_forwarded_port`
+- `$http_x_forwarded_prefix`
+- `$http_x_forwarded_proto`
+- `$https`
+- `$http2`
+- `$realip_remote_addr`
+- `$realip_remote_port`
+- `$remote_port`
+- `$request_length`
+- `$request_method`
+- `$request_time`
+- `$request_uri`
+- `$scheme`
+- `$server_addr`
+- `$server_port`
+- `$ssl_cipher`
+- `$ssl_client_raw_cert`
+- `$ssl_client_verify`
+- `$ssl_protocol`
+- `$ssl_server_name`
+- `$upstream_http_connection`
+- `$upstream_http_trailer`
+- `$upstream_http_upgrade`
+- `$upstream_status`
+
 See [resty.kong.var.patch\_metatable](#restykongvarpatch_metatable) on how to enable
 indexed variable access.