Add documentation on usage

README.md

+# Gakeshadeba
+
+*(ka. to be put in cache)*
+
+Simple ShaCache and ShaDir implementation in plain Python.
+For now, support is limited to **binary releases**.
+
+## Server usage
+
+The server script is meant to be easy to use. It requires four main things:
+
+1. a configuration file for the cache server itself;
+2. a directory where all binary releases (`.tar.gz` files) are stored;
+3. a list of software releases, in JSON format;
+4. a key used to sign software releases.
+
+After everything is set up, the server can be launched using:
+
+```bash
+pip install -r requirements.txt
+python3 server.py -c /path/to/config.ini`
+```
+
+### Configuring the server
+
+Python's `configparser` module is used, therefore, create a `config.ini` file with the following content (adapted to your needs):
+
+```ini
+[server]
+host = 192.168.32.10
+port = 5050
+
+[gakeshadeba]
+content_directory = /mnt/device/software
+sr_list_path = /mnt/device/miscellaneous/sr-list.json
+signing_key_path = /mnt/device/miscellaneous/my.key
+```
+
+### Adding software releases
+
+The `sr-list.json` file must be created and maintained manually for now, it contains several entries like:
+
+```json
+{
+	"cache_file": "re6stnet-1.0.349-ubuntu-22.04.tar.gz",
+	"software_release": "https://lab.nexedi.com/nexedi/slapos/raw/1.0.349/software/re6stnet/software.cfg",
+	"software_root": "/opt/slapgrid",
+	"multiarch": "x86_64-linux-gnu",
+	"os": ["ubuntu", "22.04", "jammy"]
+}
+```
+

requirements.txt

+blinker==1.7.0
+cffi==1.16.0
+click==8.1.7
+cryptography==41.0.7
+Flask==3.0.0
+itsdangerous==2.1.2
+Jinja2==3.1.2
+MarkupSafe==2.1.3
+pycparser==2.21
+pyOpenSSL==23.3.0
+Werkzeug==3.0.1