Doc: List required packages for linting through VSCode

This feature has always been available but now requires additional
global packages to be installed.

VSCode's ESLint extension does not run inside the project's environment.

README-DEVELOPER.md

@@ -67,7 +67,11 @@ One can check coding style using
 ``` sh
 npm run lint
 ```
-Install the recommended extensions to automatically fix linting errors when possible.
+Install the recommended extensions to automatically fix linting errors through VSCode.
+You will also need to install the `eslint` dependencies globally:
+``` sh
+npm install -g eslint typescript-eslint
+```
 
 ### Unit tests