feat: add readme

README.md

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+[![status-badge](https://ci.batsense.net/api/badges/105/status.svg)](https://ci.batsense.net/repos/105)
+---
+
+# dcache: Distributed, Highly Available cache implementation for mCaptcha
+
+## Overview
+
+-   Uses Raft consensus algorithm via [openraft](https://crates.io/crates/openraft)
+-   GRPC via [tonic](https://crates.io/crates/tonic)
+
+## Tips
+
+We recommend running at least three instances of dcache in your
+deployment.
+
+**NOTE: Catastrophic failure will occur when n/2 + 1 instances are
+down.**
+
+## Usage
+
+## Firewall configuration
+
+dcache uses a single, configurable port for both server-to-server and client-to-server
+communications. Please open that port on your server.
+
+## Launch
+
+```bash
+dcache --id 1 \
+--http-addr 127.0.0.1:9001 \
+--introducer-addr 127.0.0.1:9001 \
+--introducer-id 1 \
+--cluster-size 3
+```
+
+### Options
+
+| Name              | Purpose                                                     |
+| ----------------- | ----------------------------------------------------------- |
+| --id              | Unique integer to identify node in network                  |
+| --http-addr       | Socket address to bind and listen for connections           |
+| --introducer-addr | Socket address of introducer node; required to join network |
+| --intdocuer-id    | ID of the introducer node; required to join network         |
+| --cluster-size    | Total size of the cluster                                   |