docs: add local vuepress instructions for theme ejection (#2590)

Figuring this out was the first pain point for me in getting started with VuePress. So I think maybe above bar to add a line like this.

packages/docs/docs/theme/default-theme-config.md

@@ -559,7 +559,7 @@ This will render `.vuepress/components/SpecialLayout.vue` for the given page.
 
 ## Ejecting
 
-You can copy the default theme source code into `.vuepress/theme` to fully customize the theme using the `vuepress eject [targetDir]` command.
+You can copy the default theme source code into `.vuepress/theme` to fully customize the theme using the `vuepress eject [targetDir]` command. If you didn't install Vuepress globally, run `./node_modules/.bin/vuepress eject`.
 
 ::: warning
 Once you eject, you are on your own and **won’t** be receiving future updates or bugfixes to the default theme even if you upgrade VuePress.