Create CONNECT_EXT.md

doc/CONNECT_EXT.md

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+# CONNECT protocol extension.
+
+This stratum protocol extension allow miner use proxy for forwarding donation if direct connections to donation servers not possible.
+
+## Extensions list
+If proxy support this extension value `connect` should be added to extension list in login request reply and optionaly `tls` if proxy support SSL/TLS connections.
+
+```json
+{
+  "id": 1, "jsonrpc": "2.0", "error": null,
+  "result": {
+    "id": "...",
+    "job": { "blob": "...", "job_id": "...", "target": "...", "id": "...", "algo": "..." },
+    "extensions" : ["algo","nicehash","connect","tls","keepalive"],
+    "status": "OK"
+  }
+}
+```
+
+## Extended login request.
+
+If miner detect current connection support this extension and donation time is come, miner will create new connection to proxy with one extra field `url` in login request.
+
+```json
+{
+  "id": 1, "jsonrpc": "2.0", "method": "login",
+  "params": {
+    "login": "...", "pass": "...", "agent": "...", "algo": [...],
+    "url": "stratum+ssl://donate.ssl.xmrig.com:443"
+  }
+}
+```
+
+Proxy will perform new connection to requested url, fields `user`, `rigid`, `agent`, `algo` will be forwarded as is to upstream donation server.
+