Add simple READMEs for SDK test clients

crates/sdk/tests/connect_disconnect_client/README.md

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+This test client is used with two modules:
+
+- [`sdk-test-connect-disconnect`](/modules/sdk-test-connect-disconnect)
+- [`sdk-test-connect-disconnect-cs`](/modules/sdk-test-connect-disconnect-cs)
+
+Currently, the bindings are generated using only one of those two modules,
+chosen arbitrarily on each test run.
+The two tests which use this client, 
+`connect_disconnect_callbacks` and `connect_disconnect_callbacks_csharp`,
+are not intended to test code generation.
+
+The goal of the two tests is to verify that module-side `connect` and `disconnect` events
+fire when an SDK connects or disconnects via WebSocket,
+and that the client can observe mutations performed by those events.
+
+To (re-)generate the `module_bindings`, from this directory, run:
+
+```sh
+mkdir -P src/module_bindings
+spacetime generate --lang rust                                     \
+    --out-dir src/module_bindings                                  \
+    --project-path ../../../../modules/sdk-test-connect-disconnect
+```

crates/sdk/tests/test-client/README.md

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+This test client is used with the module [`sdk-test`](/modules/sdk-test).
+
+It is invoked by a majority of the SDK tests,
+and is responsible for testing that 
+serialization, deserialization, type dispatch, and client-side callbacks
+work as expected.
+
+To (re-)generate the `module_bindings`, from this directory, run:
+
+```sh
+mkdir -p src/module_bindings
+spacetime generate --lang rust --out-dir src/module_bindings --project-path ../../../../modules/sdk-test
+```