Add documentation (#110)

docs/configuration/index.md

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+# Configuration
+
+Opengist provides flexible configuration options through either a YAML file and/or environment variables.
+You would only need to specify the configuration options you want to change — for any config option left untouched, 
+Opengist will simply apply the default values.
+
+The [configuration cheat sheet](cheat-sheet.md) lists all available configuration options.
+
+
+## Configuration via YAML file
+
+The configuration file must be specified when launching the application, using the `--config` flag followed by the path 
+to your YAML file.
+
+```shell
+./opengist --config /path/to/config.yml
+```
+
+You can start by copying and/or modifying the provided [config.yml](/config.yml) file.
+
+
+## Configuration via Environment Variables
+
+Usage with Docker Compose :
+
+```yml
+services:
+  opengist:
+    # ...
+    environment:
+      OG_LOG_LEVEL: "info"
+      # etc.
+```
+Usage via command line :
+
+```shell
+OG_LOG_LEVEL=info ./opengist
+```