add doc under external

external/README.md

@@ -0,0 +1,7 @@
+# Note
+
+The `external` dir in the project exists for historical reasons. We will migrate to use go mod to maintain 3rd party libraries.
+
+Fow now, all modules under external are used by "quic-go" to support quic protocol, and can't migrate without a breaking change.
+
+The plan is that we will remove the whole `external` dir when `quic-go` is tested to be matured enough in production.