docs(typescript-plugin): add README (#3974)

Fixes: #3925

packages/typescript-plugin/README.md

@@ -0,0 +1,33 @@
+# typescript plugin
+
+This is a plug-in for `tsserver` or `typescript-language-server`. It must be
+installed in a file-system location accessible by the language server or in the
+`node_modules` directory of projects being edited.
+
+The LSP client must be configured to explicitly enable this plug-in. This is
+done by passing `initializationOptions` with the appropriate [`plugins`]
+configuration to the language server:
+
+[`plugins`]: https://github.com/typescript-language-server/typescript-language-server/blob/b224b878652438bcdd639137a6b1d1a6630129e4/docs/configuration.md?plain=1#L27-L31
+
+```json
+"initializationOptions":  {
+    "plugins": [
+        {
+          "name": "@vue/typescript-plugin",
+          "location": "/usr/local/lib/node_modules/@vue/typescript-plugin",
+          "languages": ["javascript", "typescript", "vue"],
+        },
+    ],
+  },
+```
+
+The `languages` field must specify file-types for which the plug-in will be
+enabled. If the plug-in package is installed in the local `node_modules`, the
+`location` field may contain any arbitrary string, but MUST be present.
+
+## Client-specific configuration
+
+- For neovim, see the [details on configuring `tsserver`][nvim].
+
+[nvim]: https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#vue-support