npm.md: additional module listing use case

Added a use case for listing all globally installed modules with a tree depth of 0.

pages/common/npm.md

@@ -27,6 +27,10 @@
 
 `npm list`
 
+- List a tree of globally installed modules:
+
+`npm list -g --depth=0`
+
 - Interactively create a package.json file:
 
 `npm init`