[webpack readme] Add how to get languages/features

I had to do a lot of digging in the source code just to know what strings I can use in the features list. Adding this allows users to find out for themselves. I considered adding the list here but I don't know if it might change depending on your build.

webpack-plugin/README.md

@@ -78,8 +78,12 @@ Options can be passed in to `MonacoWebpackPlugin`. They can be used to generate
   | handlebars | html         |
   | scss, less | css          |
 
+  To view a list of all available languages, you can run `import metadata from 'monaco-editor/esm/metadata'; console.log(metadata.languages);`.
+
 - `features` (`string[]`) - include only a subset of the editor features. By default, all features shipped with the `monaco-editor` will be included. Instead of enumerating included features, it is also possible to exclude certain default features prefixing them with an exclamation mark '!'.
 
+  To view a list of all available features, you can run `import metadata from 'monaco-editor/esm/metadata'; console.log(metadata.features);`.
+
 - `globalAPI` (`boolean`) - specify whether the editor API should be exposed through a global `monaco` object or not. This option is applicable to `0.22.0` and newer version of `monaco-editor`. Since `0.22.0`, the ESM version of the monaco editor does no longer define a global `monaco` object unless `global.MonacoEnvironment = { globalAPI: true }` is set ([change log](https://github.com/microsoft/monaco-editor/blob/main/CHANGELOG.md#0220-29012021)).
   - default value: `false`.