doc: add lua_upstream_skip_openssl_default_verify description

Author: willmafh

README.md

@@ -167,6 +167,7 @@ behavior.
 * [lua_ssl_trusted_certificate](https://github.com/openresty/lua-nginx-module#lua_ssl_trusted_certificate)
 * [lua_ssl_verify_depth](https://github.com/openresty/lua-nginx-module#lua_ssl_verify_depth)
 * [lua_ssl_conf_command](https://github.com/openresty/lua-nginx-module#lua_ssl_conf_command)
+* [lua_upstream_skip_openssl_default_verify](https://github.com/openresty/lua-nginx-module#lua_upstream_skip_openssl_default_verify)
 * [lua_check_client_abort](https://github.com/openresty/lua-nginx-module#lua_check_client_abort)
 * [lua_max_pending_timers](https://github.com/openresty/lua-nginx-module#lua_max_pending_timers)
 * [lua_max_running_timers](https://github.com/openresty/lua-nginx-module#lua_max_running_timers)