Update quick start guide to reference tutorial files

Quick start guide references tutorial files that were destroyed
in gn migration:
https://chromium-review.googlesource.com/c/chromium/src/+/1129822
The files were resurrected in chromium repo and this change
updates the quick start guide to reference the tutorial files in
the chromium repo.

See https://bugs.chromium.org/p/chromium/issues/detail?id=923492#c1

Bug: 41
Change-Id: I9702694320d196d4c4369dd94c569fa609a55473
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/3780
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>

docs/quick_start.md

diff --git a/docs/quick_start.md b/docs/quick_start.md
index 4e1bdaf..d3941c5 100644
--- a/docs/quick_start.md
+++ b/docs/quick_start.md
@@ -2,6 +2,10 @@
 
 [TOC]
 
+Note: the gn repo has been ported from chromium, and the tutorial within this
+document has not yet been updated to reflect this. Building the tutorial files
+will require the chromium tree, even though gn is now independent of chromium.
+
 ## Running GN
 
 You just run `gn` from the command line. There is a script in
@@ -94,7 +98,8 @@
 
 ### Adding a build file
 
-Create a `tools/gn/tutorial/BUILD.gn` file and enter the following:
+Create a `tools/gn/tutorial/BUILD.gn` file [in the chromium repo](https://cs.chromium.org/chromium/src/tools/gn/tutorial/) and enter
+the following:
 
 ```
 executable("hello_world") {
@@ -134,9 +139,11 @@
 out/Default/hello_world
 ```
 
-GN encourages target names for static libraries that aren't globally
-unique. To build one of these, you can pass the label with no leading
-"//" to ninja:
+You should see "Hello, world." output to the console.
+
+Side note: GN encourages target names for static libraries that aren't globally
+unique. To build one of these, you can pass the label with its path (but no leading
+"//") to ninja:
 
 ```
 ninja -C out/Default tools/gn/tutorial:hello_world