[doc] add install instructions & usage example

README.md

@@ -2,11 +2,26 @@
 
 Mnemonic generator
 
-## CLi usage
+## Install
 
-    dup-mnemonic [OPTIONS]
+[Download debian package on releases page](https://git.duniter.org/tools/dup-mnemonic-rs/-/releases)
 
-Options :
+### Compile from sources
+
+    git clone https://git.duniter.org/tools/dup-mnemonic-rs
+    cd dup-mnemonic-rs
+    cargo build --release
+
+## Usage
+
+    $ dup-mnemonic [OPTIONS]
+
+Generate a french mnemotic with 12 words:
+
+    $ dup-mnemonic -l fr -w 12
+    narquois horde estrade banlieue vigueur poumon subvenir entier chercher cultiver trancher froid
+
+All options :
 
     -l, --language <language>          Mnemotic language [default: en]  [possible values: en, zh_HANS, zh_HANT, fr, it,
                                        ja, ko, es]