Update README to describe configuration variables

README.md

@@ -72,6 +72,25 @@ For the WebAssembly component, you can run tests in a headless browser with
 wasm-pack test rustpad-wasm --chrome --headless
 ```
 
+## Configuration
+
+Although the default behavior of Rustpad is to store documents solely in memory
+and collect garbage after 24 hours of inactivity, this can be configured by
+setting the appropriate variables. The application server looks for the
+following environment variables on startup:
+
+- `EXPIRY_DAYS`: An integer corresponding to the number of days that inactive
+  documents are kept in memory before being garbage collected by the server
+  (default 1 day).
+- `SQLITE_URI`: A SQLite connection string used for persistence. If provided,
+  Rustpad will persist the contents of documents to a local file on disk, which
+  will enable documents to be retained between server restarts, and even after
+  they are garbage collected due to inactivity.
+- `PORT`: Which local port to listen for HTTP connections on (defaults to 3030).
+- `RUST_LOG`: Controls application server logging, see the
+  [env_logger](https://docs.rs/env_logger/#enabling-logging) docs for more
+  information.
+
 ## Deployment
 
 Rustpad is distributed as a single 4 MB Docker image, which is built