docs: Update Usage docs to include Domains in Promise support

Author: WanLinLin

docs/usage.rst

@@ -90,6 +90,8 @@ A context might also correspond to, say, a connection lifecycle or a job being h
 Notable keys that you might set include ``user``, ``tags``, and ``extra``.
 These types of extra context data are detailed more under :ref:`raven-node-additional-data`.
 
+Since ``domains`` are not supported in native Promise until Node v8.0.0, the Node LTS version ``>=8.0.0`` environment is required if you want to manage different contexts in Promises. Context for regular ``context/wrap`` and instrumentation is working prior to 8.x just fine. It'll just be skipped and will use globally set context data.
+
 Tracking Users
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