must take lua_code_cache on

I use openresty several months, just a new hand, and don't understand what happend behind lua
code.when I use this library， for convenience， I take lua_code_cache off, but I find that the logs won't be
accumulated according to the param , if I take lua_code_cache on , it will run normally. a
helper tell me that when taking lua_code_cache on, the top level variable in lua module will be cached, this
library just use this point , but there is none description. I advice you to add description on this readme, if
that has some description in lua_nginx_module, it would be better.
thanks.