Added readme

certifier/README.md

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+# POST certifier service
+A certifier service that creates certificates for a node confirming it holds a valid POST proof.
+
+The client (presumably the spacemesh node) submits a POST proof with its metadata to the certifier on /certify HTTP endpoint. The certifier validates the proof and, if valid - signs the nodeID and returns the signature. If the proof is invalid it returns a 403 status code.
+
+The client can later use this certificate to register in a poet. The poet is supposed to know the certifier's public key and verify the signature over a node ID.
+
+## Usage
+```
+Usage: certifier [OPTIONS] [COMMAND]
+
+Commands:
+  generate-keys  generate keypair and write it to standard out. the keypair is encoded as json
+  help           Print this message or the help of the given subcommand(s)
+
+Options:
+  -c, --config <CONFIG>  [env: CERTIFIER_CONFIG_PATH=] [default: config.yml]
+  -h, --help             Print help
+  -V, --version          Print version
+```
+
+### running a service
+To run a service, execute the certifier w/o additional command and provide a path to config (either via --config or CERTIFIER_CONFIG_PATH env variable).
+
+#### Configuration
+The config structure is defined [here](src/configuration.rs). An example config:
+
+```yaml
+listen: "127.0.0.1:8080"
+signing_key: <BASE64-encoded ed25519 private key>
+post_cfg:
+  k1: 26
+  k2: 37
+  k3: 37
+  pow_difficulty: "000dfb23b0979b4b000000000000000000000000000000000000000000000000"
+
+init_cfg:
+  min_num_units: 4
+  max_num_units: 99999
+  labels_per_unit: 4294967296
+  scrypt:
+    n: 8192
+    r: 1
+    p: 1
+
+metrics: "127.0.0.1:9090"
+```
+
+Each field can also be provided as env variable prefixed with CERTIFIER. For example, `CERTIFIER_SIGNING_KEY`.
+
+#### Docker
+There is a docker image created to simplify deployment: `spacemeshos/certifier-service`.
+
+### Generating keys
+Run `certifier generate-keys` to obtain randomly generated new keys.
+```sh
+❯ certifier generate-keys
+{
+  "public_key": "N2QnP1E3QmrPIjHt8QPvQXThNjDVKfatr0ncttXn7+Q=",
+  "secret_key": "DoU0xoVZ1gf/FDvz4K9PldiYCCeiFPhQmWbHd+X6Yjo="
+}
+```
+
+## Log level
+The log level can be controlled via `RUST_LOG` enviroment variable. It can be set to [error, warn, info, debug, trace, off].