documented setting nginx special variables $limit_rate/$args, and reading special variables $1, $2, $3, and etc.

README.markdown

@@ -631,8 +631,14 @@ For example:
 
 That is, nginx variables cannot be created on-the-fly.
 
+Some special nginx variables like `$args` and `$limit_rate` can be assigned a value,
+some are not, like `$arg_PARAMETER`.
+
+Nginx regex group capturing variables `$1`, `$2`, `$3`, and etc, can be read by this
+interface as well, by writing `ngx.var[1]`, `ngx.var[2]`, `ngx.var[3]`, and etc.
+
 Core constants
----------------------
+--------------
 * **Context:** `rewrite_by_lua*`, `access_by_lua*`, `content_by_lua*`
 
     ngx.OK