Adding in docs

docs/src/commands/index.md

@@ -14,3 +14,23 @@ a Rust-generated WebAssembly project.
 [build]: ./build.html
 [new]: ./new.html
 [pack-pub]: ./pack-and-publish.html
+
+### Log levels
+
+By default `wasm-pack` displays a lot of useful information.
+
+You can cause it to display even *more* information by using `--verbose`, or you can silence *all* stdout by using `--quiet`.
+
+You can also use `--log-level` to have fine-grained control over wasm-pack's log output:
+
+* `--log-level info` is the default, it causes all messages to be logged.
+* `--log-level warn` causes warnings and errors to be displayed, but not info.
+* `--log-level error` causes only errors to be displayed.
+
+These flags are global flags, so they can be used with every command, and they must come *before* the command:
+
+```sh
+wasm-pack --log-level error build
+wasm-pack --quiet build
+wasm-pack --verbose build
+```