Actually include the docs for local echo

docs/local-echo-dev.md

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+# Local echo (developer docs)
+
+The React SDK provides some local echo functionality to allow for components to do something
+quickly and fall back when it fails. This is all available in the `local-echo` directory within
+`stores`.
+
+Echo is handled in EchoChambers, with `GenericEchoChamber` being the base implementation for all
+chambers. The `EchoChamber` class is provided as semantic access to a `GenericEchoChamber`
+implementation, such as the `RoomEchoChamber` (which handles echoable details of a room).
+
+Anything that can be locally echoed will be provided by the `GenericEchoChamber` implementation.
+The echo chamber will also need to deal with external changes, and has full control over whether
+or not something has successfully been echoed. 
+
+An `EchoContext` is provided to echo chambers (usually with a matching type: `RoomEchoContext`
+gets provided to a `RoomEchoChamber` for example) with details about their intended area of
+effect, as well as manage `EchoTransaction`s. An `EchoTransaction` is simply a unit of work that
+needs to be locally echoed.
+
+The `EchoStore` manages echo chamber instances, builds contexts, and is generally less semantically
+accessible than the `EchoChamber` class. For separation of concerns, and to try and keep things
+tidy, this is an intentional design decision.
+
+**Note**: The local echo stack uses a "whenable" pattern, which is similar to thenables and 
+`EventEmitter`. Whenables are ways of actioning a changing condition without having to deal
+with listeners being torn down. Once the reference count of the Whenable causes garbage collection,
+the Whenable's listeners will also be torn down. This is accelerated by the `IDestroyable` interface
+usage.
+
+## Audit functionality
+
+The UI supports a "Server isn't responding" dialog which includes a partial audit log-like
+structure to it. This is partially the reason for added complexity of `EchoTransaction`s
+and `EchoContext`s - this information feeds the UI states which then provide direct retry
+mechanisms.
+
+The `EchoStore` is responsible for ensuring that the appropriate non-urgent toast (lower left)
+is set up, where the dialog then drives through the contexts and transactions.
+