rainbow README

cmd/rainbow/README.md

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+
+`rainbow`: atproto Firehose Fanout Service
+==========================================
+
+This is an atproto service which consumes from a firehose (eg, from a relay or PDS) and fans out events to many subscribers.
+
+Features and design points:
+
+- retains "backfill window" on local disk (using [pebble](https://github.com/cockroachdb/pebble))
+- serves the `com.atproto.sync.subscribeRepos` endpoint (WebSocket)
+- retains upstream firehose "sequence numbers"
+- does not validate events (signatures, repo tree, hashes, etc), just passes through
+- does not archive or mirror individual records or entire repositories (or implement related API endpoints)
+- disk I/O intensive: fast NVMe disks are recommended, and RAM is helpful for caching
+- single golang binary for easy deployment
+- observability: logging, prometheus metrics, OTEL traces
+
+## Running 
+
+This is a simple, single-binary Go program. You can also build and run it as a docker container (see `./Dockerfile`).
+
+From the top level of this repo, you can build:
+
+```shell
+go build ./cmd/rainbow -o rainbow-bin
+```
+
+or just run it, and see configuration options:
+
+```shell
+go run ./cmd/rainbow --help
+```