Add a short README for the c++ dataflow example

examples/c++-dataflow/README.md

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+# Dora C++ Dataflow Example
+
+This example shows how to create dora operators and custom nodes with C++.
+
+Dora does not provide a C++ API yet, but we can create adapters for either the C or Rust API. The `operator-rust-api` and `node-rust-api` folders implement an example operator and node based on dora's Rust API, using the `cxx` crate for bridging. The `operator-c-api` and `node-c-api` show how to create operators and nodes based on dora's C API. Both approaches work, so you can choose the API that fits your application better.