docs: Update proxy usage info - add mention of proxy-chain (#1183)

Author: jancurn

sources/platform/proxy/usage.md

@@ -198,3 +198,6 @@ The typical issues behind these codes are:
 * `594`, `595` and `596` may occur due to connection loss.
 * `597` indicates incorrect upstream credentials.
 * `599` is a generic error, where the above is not applicable.
+
+  Note that the Apify Proxy is based on the [proxy-chain](https://github.com/apify/proxy-chain) open-source NPM package developed and maintained by Apify.
+  You can find the details of the above errors and their implementation there.