docs: document eject

Author: yyx990803

docs/default-theme-config/README.md

@@ -31,6 +31,8 @@ footer: MIT Licensed | Copyright © 2018-present Evan You
 
 Any additional content after the front matter will be parsed as normal markdown and rendered after the features section.
 
+If you want to use a completely custom homepage layout, you can also use a [Custom Layout](#custom-layout-for-specific-pages).
+
 ## Navbar Links
 
 You can add links to the navbar via `themeConfig.nav`:
@@ -223,3 +225,7 @@ layout: SpecialLayout
 ```
 
 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. Note, however, once you eject, you are on your own and won't be receiving future updates or bug fixes to the default theme even if you upgrade VuePress.

docs/guide/custom-themes.md

@@ -79,3 +79,7 @@ module.exports = {
 ```
 
 VuePress will attempt to locate and use `node_modules/vuepress-theme-awesome/Layout.vue`.
+
+## Customizing the Default Theme
+
+The `vuepress eject [targetDir]` command will copy the default theme source code into `.vuepress/theme` to allow complete customization. Note, however, once you eject, you are on your own and won't be receiving future updates or bug fixes to the default theme even if you upgrade VuePress.