feat: added flexible configuration system using figment

README.md

@@ -11,3 +11,24 @@ Gpodder, suitable for low-power devices.
     * Devices API
     * Subscriptions API
     * Episode actions API
+
+## Configuration
+
+Otter is configurable via a TOML config file, environment variables and CLI
+arguments thanks to the [Figment](https://docs.rs/figment/latest/figment/)
+library. Each variable in the configuration file below can be provided via an
+environment variable (prefixed using `OTTER_`) or a CLI argument.
+
+```toml
+data_dir = "./data"
+domain = "127.0.0.1"
+port = 8080
+```
+
+Otter can be told to read a config variable using the `--config/-c` flag (see
+the `help` command for other useful CLI commands). Each variable needs to be
+provided either from the config file, as an environment variable (e.g.
+`OTTER_DATA_DIR`) or a CLI argument (e.g. `--data-dir`).
+
+If a variable is provided in multiple ways, environment variables overwrite
+configuration file variables, and CLI arguments overwrite both.