Add README (#1)

README.md

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+# OpenID Connect Identity Provider for Sign-In with Ethereum
+
+## Getting Started
+
+### Dependencies
+
+Redis, or a Redis compatible database (e.g. MemoryDB in AWS), is required.
+
+### Starting the IdP
+
+The Docker image is available at `ghcr.io/spruceid/siwe_oidc:0.1.0`. Here is an
+example usage:
+```bash
+docker run -p 8000:8000 -e SIWEOIDC_ADDRESS="0.0.0.0" -e SIWEOIDC_REDIS_URL="redis://redis" ghcr.io/spruceid/siwe_oidc:latest
+```
+
+It can be configured either with the `siwe-oidc.toml` configuration file, or
+through environment variables:
+* `SIWEOIDC_ADDRESS` is the IP address to bind to.
+* `SIWEOIDC_REDIS_URL` is the URL to the Redis instance.
+* `SIWEOIDC_BASE_URL` is the URL you want to advertise in the OIDC configuration
+  (e.g. `https://oidc.example.com`).
+* `SIWEOIDC_RSA_PEM` is the signing key, in PEM format. One will be generated if
+  none is provided.
+
+### OIDC Functionalities
+
+The current flow is very basic -- after the user is authenticated you will
+receive an Ethereum address as the subject (`sub` field).
+
+For the core OIDC information, it is available under
+`/.well-known/openid-configuration`.
+
+### TODO Items
+
+* Additional information, from native projects (e.g. ENS domains), to more
+  traditional ones (e.g. email).
+
+## Development
+
+A Docker Compose is available to test the IdP locally with Keycloak.
+
+1. You will first need to run:
+```bash
+docker-compose up -d
+```
+
+2. And then edit your `/etc/hosts` to have `siwe-oidc` point to `127.0.0.1`.
+   This is so both your browser, and Keycloak, can access the IdP.
+
+3. In Keycloak, you will need to create a new IdP. You can use
+   `http://siwe-oidc:8000/.well-known/openid-configuration` to fill the settings
+   automatically. As for the client ID/secret, you can use `sdf`/`sdf`.