doc(readme): description & usage

README.md

-gcli
+# gcli
+
+CLI client for [Duniter-V2S](https://git.duniter.org/nodes/rust/duniter-v2s/).
+
+## Usage
+
+Send 10 ĞD from Alice to Ferdie:
+
+	cargo run -- --url ws://localhost:9944 --secret //Alice transfer 1000 5CiPPseXPECbkjWCa6MnjNokrgYjMqmKndv2rSnekmSK2DjL
+
+If using a different runtime, update the metadata for the client to compile:
+
+	subxt metadata -f bytes > res/metadata.scale