Add ADR about websocket asynchronous API

docs/adr/0003-asynchronous-duplex-api.md

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+# 3. Asynchronous Duplex Client API
+
+Date: 2021-06-07
+
+## Status
+
+Accepted
+
+## Context
+
+The [_reactive_ nature of the Hydra node](0001-record-architecture-decisions) means _Request-response_ paradigm is not a good fit to provide a _Client API_: Clients input a _stream_ of _commands_ to a node which in turns issues a stream of _responses_ representing the observable outcome of previous commands and interaction with peers in the network.
+
+## Decision
+
+* The main API is exposed using the [Websocket](https://datatracker.ietf.org/doc/html/rfc6455) protocol with inputs and outputs conforming to some textual representation of `Command`s and `Result`s.
+
+## Consequences
+
+* Client software should be implemented accordingly and most notably cannot rely on any kind of synchronous response to give feedback to potential users or higher-level clients.
+* Clients can receive "responses" decorrelated from any requests, for example as the consequence of actions carried out by other nodes