voluxcli: docs: add installation and usage instructions

voluxcli/README.md

 # Volux CLI
 
+See https://gitlab.com/volux/volux
+
+## Installation
+
+```bash
+pip install voluxcli
+```
+
+## Usage
+
+See `volux --help` for usage information.
+
 <!-- TODO: add basic documentation -->
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-<!-- TODO: add install instructions -->