Add documentation for using RUST_LOG

Author: kleinesfilmroellchen

docs/content/documentation/getting-started/cli-usage.md

@@ -139,3 +139,13 @@ You can disable this behavior by exporting one of the following two environment
 To force the use of colors, you can set the following environment variable:
 
 - `CLICOLOR_FORCE=1`
+
+## Extra information
+
+Zola can provide detailed logging about its behavior via the `RUST_LOG` variable:
+
+- To see debug information from Zola, set `RUST_LOG=zola=debug`
+- To see debug information from everywhere (including Zola's dependencies), set `RUST_LOG=debug`. *Note*: The output will be **very noisy**, use with caution.
+- To disable all log output entirely, set `RUST_LOG=off`.
+
+See the [env_logger documentation](https://docs.rs/env_logger/0.11.8/env_logger/#enabling-logging) for a full reference on `RUST_LOG`.