chore: move plugin docs

docs/Plugin.md

@@ -0,0 +1,75 @@
+# Plugin Development Guide
+
+## Core Concepts
+
+There are two major parts of the system:
+
+- `@vue/cli`: globally installed, exposes the `vue create <app>` command;
+- `@vue/cli-service`: locally installed, exposes the `vue-cli-service` commands.
+
+Both utilize a plugin-based architecture.
+
+### Creator
+
+[Creator][1] is the class created when invoking `vue create <app>`. Responsible for prompting for preferences, invoking generators and installing dependencies.
+
+### Service
+
+[Service][4] is the class created when invoking `vue-cli-service <command> [...args]`. Responsible for managing the internal webpack configuration, and exposes commands for serving and building the project.
+
+### Plugin
+
+Plugins are locally installed into the project as dependencies. `@vue/cli-service`'s [built-in commands][5] and [config modules][6] are also all implemented as plugins. This repo also contains a number of plugins that are published as individual packages.
+
+A plugin should export a function which receives two arguments:
+
+- A [PluginAPI][7] instance
+- Project local options specified in `vue.config.js`, or in the `"vue-cli"` field in `package.json`.
+
+The API allows plugins to extend/modify the internal webpack config for different environments and inject additional commands to `vue-cli-service`. Example:
+
+``` js
+module.exports = (api, projectOptions) => {
+  api.chainWebpack(webpackConfig => {
+    // modify webpack config with webpack-chain
+  })
+
+  api.configureWepback(webpackConfig => {
+    // modify webpack config
+    // or return object to be merged with webpack-merge
+  })
+
+  api.regsiterCommand('test', args => {
+    // register `vue-cli-service test`
+  })
+}
+```
+
+### Generator
+
+A plugin published as a package can also contain a `generator.js` file or a `generator` directory with `index.js`. The generator inside a plugin will be invoked after the plugin is installed.
+
+A generator should export a function which receives a [GeneratorAPI][3] instance as the only argument. The API allows a generator to inject additional dependencies or fields into `package.json` and add files to the project. Example:
+
+``` js
+module.exports = (api, options) => {
+  api.extendPackage({
+    scripts: {
+      test: 'vue-cli-service test'
+    }
+  })
+
+  api.render('./template')
+}
+```
+
+#### Important Development Note
+
+A plugin with a generator that injects additional dependencies other than packages in this repo (e.g. `chai` is conditionally injected by `@vue/cli-plugin-unit-mocha-webpack/generator/index.js`) should have those dependencies listed in its "devDependencies" field. This ensures that the package always exist in this repo's root `node_modules` so that we don't have to reinstall them on every test.
+
+[1]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli/lib/Creator.js
+[3]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli/lib/GeneratorAPI.js
+[4]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli-service/lib/Service.js
+[5]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli-service/lib/commands
+[6]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli-service/lib/config
+[7]: https://github.com/vuejs/vue-cli/tree/next/packages/@vue/cli-service/lib/PluginAPI.js