service/rpc1: add Go Reference doc (#3779)

Author: alexandear

service/rpc1/doc.go

@@ -0,0 +1,5 @@
+// Package rpc1 implements version 1 of Delve's API and is only
+// kept for backwards compatibility.
+// client.go is the old client code used by Delve's frontend (delve/cmd/dlv) and
+// is preserved for backwards compatibility with integration tests.
+package rpc1