doc: add usage documentation

README.md

 # duniter-mini-client
 
+A simple Duniter client written in Rust.
+
+Just for training with Rust.
+
 ## Requirements
 
 - `sudo apt install libssl-dev`
@@ -10,9 +14,45 @@
 
 ## Usage
 
+### Target node
+
     duniter-mini-client lookup cgeek
 
 By default, mini-client will target `https://g1-test.duniter.org`. To use another node, change `DUNITER_NODE` environment variable :
 
     DUNITER_NODE=https://g1.cgeek.fr duniter-mini-client lookup cgeek
 
+Or : 
+
+    export DUNITER_NODE=https://g1.cgeek.fr
+
+### Commands
+
+#### `lookup`
+
+Performs a BMA /wot/lookup HTTP request to find identities both on mempool and blockchain.
+
+    duniter-mini-client lookup cgeek
+
+#### `certify`
+
+> Requires a keyring file (see Options > Keyring section below)
+
+Certify and identity by creating the certification and submitting it to the network.
+
+    duniter-mini-client certify cgeek
+
+#### `adhere`
+
+> Requires a keyring file (see Options > Keyring section below)
+
+Adhere to or renew your membership to the currency.
+
+    duniter-mini-client adhere cgeek
+
+## Options
+
+The client uses environment variables to set specific options:
+
+* `DUNITER_NODE`: the network node (with BMA enabled) which will receive the HTTP requests
+* `DUNITER_KEYRING_PATH`: path to the keyring generated using `duniter-mini-client keyring <salt> <passwd>` command
\ No newline at end of file