Add notes about styleguide command in README

To make it a bit more obvious.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

README.md

@@ -51,6 +51,12 @@ If you want to work on improving the components it’s best to run the latest co
     - In the repository of an app do `npm link @nextcloud/vue` (you need to re-link any time you do `npm ci` in the app)
 4. Then build the app with: `npm run build` (or watch for changes with `npm run watch`)
 
+### Styleguide
+
+When developing new components or extending compnents, make sure to also have some bits of related documentation like examples, where applicable.
+To test components and the documentation in that context, you can run `npm run styleguide` to run a local server that serves the style guide
+with all the components.
+
 ### Using vue-devtools in Firefox
 
 If you want to use [vue-devtools](https://github.com/vuejs/vue-devtools) in Firefox, you need to patch your nextcloud instance as follow: