docs: add a note for users behind a proxy

Author: robin850

README.md

@@ -14,6 +14,8 @@ On starting a new instance of the memory server, if the binary cannot be found,
 
 This package automatically downloads binaries from [https://fastdl.mongodb.org/](https://fastdl.mongodb.org/) according to your operating system. You can see all available versions by the following links [Linux](https://www.mongodb.org/dl/linux) (Ubuntu, RHEL, Debian, SUSE, Amazon), [OSX](https://www.mongodb.org/dl/osx), [Win](https://www.mongodb.org/dl/win32).
 
+> If your network is behind a proxy, make sure that it is configured through the `HTTPS_PROXY` or `HTTP_PROXY` environment variable.
+
 Every `MongoMemoryServer` instance creates and starts fresh MongoDB server on some free port. You may start up several mongod simultaneously. When you terminate your script or call `stop()` MongoDB server(s) will be automatically shutdown.
 
 Perfectly [works with Travis CI](https://github.com/nodkz/graphql-compose-mongoose/commit/7a6ac2de747d14281f9965f418065e97a57cfb37) without additional `services` and `addons` options in `.travis.yml`.