Document environment variable configuration

README.md

@@ -36,6 +36,8 @@ docker run \
 
 ## Configuration
 
+RustiCal can either be configured using a TOML file or using environment variables.
+
 You can generate a default `config.toml` with
 
 ```sh
@@ -48,6 +50,24 @@ rustical gen-config
 
 You'll have to set your database path to something like `/var/lib/rustical/db.sqlite3`.
 
+### Environment variables
+
+The options in `config.toml` can also be configured using environment variables. Names translate the following:
+
+```toml
+[data_store.toml]
+path = "asd"
+```
+
+becomes `RUSTICAL_DATA_STORE__TOML__PATH`.
+Every variable is
+
+- uppercase
+- prefixed by `RUSTICAL_`
+- Dots become `__`
+
+### Users and groups
+
 Next, configure the principals by creating a file specified in `auth.toml.path` (by default `/etc/rustical/principals.toml`) and inserting your principals:
 
 ```toml