Add readme.

README.md

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+# Ruaft: Raft implemented in Rust
+
+`Ruaft` is a Rust version of [the Raft consensus protocol](https://raft.github.io/). The name is clearly made up.
+
+At the moment it is a gigantic class lives in a single file. It exposes 4 APIs, to create a Raft instance (`new()`),
+start a new contract (`start()`), get the current status (`get_status()`) and shut it down (`kill()`). When a consensus
+is reached, it notifies the clients via an `apply_command` callback. It saves its internal state to local disk via a
+`persister`, which is supplied at creation.
+
+There are also
+two half-APIs: `process_append_entries()` and `process_request_vote()`, serving requests from other Raft instances of
+the same setup.
+
+The implementation is thoroughly tested. I copied the tests from an obvious source. To avoid being indexed by a search
+engine, I will not name the source. The testing framework from the same source is also translated from the original Go
+version. The code can be found at [`labrpc`](https://github.com/ditsing/labrpc) repo.
+
+## Next steps
+- [ ] Split into multiple files
+- [ ] Add public documentation
+- [ ] Add a proper RPC interface to all public methods
+- [ ] Benchmarks