docs: expand top-level README (#8)

Author: kilianglas

README.md

@@ -1,2 +1,36 @@
 # ohttp-tools
-OHTTP gateway server image and tooling (load tests, probes) shared across services.
+
+Tooling and container images for running [Oblivious HTTP (RFC 9458)](https://www.rfc-editor.org/rfc/rfc9458.html)
+gateways. Currently used by World ID; structured to be reusable for any service
+that adopts OHTTP.
+
+## Contents
+
+- [`ohttp-probe/`](./ohttp-probe) — Go CLI for probing and load-testing OHTTP
+  gateways. See [`ohttp-probe/README.md`](./ohttp-probe/README.md) for usage.
+- `.github/workflows/build-ohttp-gateway.yml` — repackages
+  [`cloudflare/privacy-gateway-server-go`](https://github.com/cloudflare/privacy-gateway-server-go)
+  at a pinned version into an org-published image.
+- `.github/workflows/build-ohttp-probe.yml` — builds and publishes the probe
+  image from this repo.
+
+## Published images
+
+Both images are public on GHCR (anonymous pull works):
+
+| Image | Source | Tags |
+|---|---|---|
+| `ghcr.io/worldcoin/ohttp-tools/ohttp-gateway` | `cloudflare/privacy-gateway-server-go` (pinned via `VERSION` in the build workflow) | `latest`, `<version>` (e.g. `v0.0.3`) |
+| `ghcr.io/worldcoin/ohttp-tools/ohttp-probe` | this repo, `ohttp-probe/Dockerfile` | `sha-<full-sha>` |
+
+Both images are `linux/amd64` only.
+
+To bump the gateway version, edit `VERSION` in
+`.github/workflows/build-ohttp-gateway.yml` and merge — the workflow rebuilds
+on `main`.
+
+## Local development
+
+The probe is a self-contained Go module — see
+[`ohttp-probe/README.md`](./ohttp-probe/README.md) for build and command
+reference.