[DOC] - documents dependency to lua-rest-http

Author: ddragosd

README.md

@@ -25,8 +25,9 @@ Description
 ===========
 
 This library requires an nginx build with OpenSSL,
-the [ngx_lua module](http://wiki.nginx.org/HttpLuaModule), [LuaJIT 2.0](http://luajit.org/luajit.html) and
-[api-gateway-hmac](https://github.com/adobe-apiplatform/api-gateway-hmac) module.
+the [ngx_lua module](http://wiki.nginx.org/HttpLuaModule), [LuaJIT 2.0](http://luajit.org/luajit.html),
+[api-gateway-hmac](https://github.com/adobe-apiplatform/api-gateway-hmac) module, and
+[lua-resty-http](https://github.com/pintsized/lua-resty-http) module.
 
 ### AWS V4 Signature
 This library supports the latest AWS V4 signature which means you can use any of the latest AWS APIs without any problem.