[torque] Update README.md for the extension to include build step

R=mathias@chromium.org

Bug: v8:8880
Change-Id: I6954ede6669f19d970ad4257fa4699da2db62fd3
Reviewed-on: https://chromium-review.googlesource.com/c/1495982Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59969}

tools/torque/vscode-torque/README.md

@@ -4,14 +4,22 @@ This extension adds language support for [the Torque language used in V8](https:
 
 ## Installation
 
-Since the extension is not published to the marketplace, the easiest way to
-install the extension is to symlink the extension to your local extension
-directory:
+Since the extension is currently not published to the marketplace, the easiest way to
+install the extension is to symlink it to your local extension directory:
 
 ```
 ln -s $V8/tools/torque/vscode-torque $HOME/.vscode/extensions/vscode-torque
 ```
 
+Additionally, for advanced language server features, the extension needs to be built
+locally (the syntax highlighting does not require this step). The following needs to be run
+everytime the extension is updated:
+
+```
+cd $V8/tools/torque/vscode-torque
+npm install
+```
+
 ### Language server
 
 The language server is not built by default. To build the language server manually:

tools/torque/vscode-torque/package.json

 {
     "name": "vscode-torque",
-    "displayName": "Torque syntax support",
-    "description": "Basic Torque syntax highlighting support",
+    "displayName": "Torque language support",
+    "description": "Syntax highlighting and language server for the V8 Torque programming language",
     "version": "0.0.1",
     "publisher": "szuend",
     "engines": {