docs(ui): WIP docs

docs/plugin-dev-ui.md

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+# Plugin UI Development Guide
+
+> Work-in-progress documentation
+
+## Plugin Info
+
+When used in the UI, your plugin can show additional information to make it more discoverable and recognizable.
+
+### Logo
+
+You can put a `logo.png` file in the root directory of the folder that will be published on npm. It will be displayed in several places:
+ - When searching for a plugin to install
+ - In the installed plugin list
+
+<p align="center">
+  <img width="911px" src="https://raw.githubusercontent.com/vuejs/vue-cli/dev/docs/plugins.png">
+</p>
+
+The logo should be a square non-transparent image (ideally 84x84).
+
+### Discoverability
+
+For better discoverability when a user searches for your plugin, put keywords describing your plugin in the `description` field of the plugin `package.json` file.
+
+Example:
+
+```json
+{
+  "name": "vue-cli-plugin-apollo",
+  "version": "0.7.7",
+  "description": "vue-cli 3 plugin to add Apollo and GraphQL"
+}
+```
+
+You should add the url to the plugin website or repository in the `homepage` or `repository` field so that a 'More info' button will be displayed in your plugin description:
+
+```json
+{
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/Akryum/vue-cli-plugin-apollo.git"
+  },
+  "homepage": "https://github.com/Akryum/vue-cli-plugin-apollo#readme"
+}
+```
+
+## UI API
+
+The cli-ui exposes an API that allows augmenting the project configurations and tasks, as well as sharing data and communicating with other processes.
+
+### UI files
+
+Inside each installed vue-cli plugins, the cli-ui will try to load an optional `ui.js` file in the root folder of the plugin. It will also try to load a `vue-cli-ui.js` file in the user project root so the UI can be manually extended on a per-project basis (also useful to quickly prototype a plugin).
+
+The file should export a function which gets the api object as argument:
+
+```js
+module.exports = api => {
+  // Use the API here...
+}
+```
+
+### Project configurations
+
+### Project tasks
+
+### Client addon
+
+### Shared data
+
+### Plugin actions
+
+### IPC
+
+### Custom views
+
+### Public static files
+
+Any file in an optional `ui-public` folder in the root of the plugin package folder will be exposed to the `/_plugin/:id/*` HTTP route.
+
+For example, if you put a `my-logo.png` file into the `my-package/ui-public/` folder, it will be available with the `http://localhost:8000/_plugin/my-package/my-logo.png` URL when the cli-ui loads the plugin.