Fix broken link and add cross-compiling to quick start

tools/gn/docs/quick_start.md

Index: tools/gn/docs/quick_start.md
diff --git a/tools/gn/docs/quick_start.md b/tools/gn/docs/quick_start.md
index 10f99ca7359f26d99ee178fec342f074254ff8e8..e6840a9432576804b3a5e9d0af5a1a4dbbd50e7e 100644
--- a/tools/gn/docs/quick_start.md
+++ b/tools/gn/docs/quick_start.md
@@ -50,6 +50,22 @@ on how to use these in your code. (Note that you have to specify the
 build directory for this command because the available arguments can
 change according to what's set.
 
+## Cross-compiling to a target OS or architecture
+
+Run `gn args out/Default` (substituting your build directory as needed) and
+add one or more of the following lines for common cross-compiling options.
+
+```
+target_os = "chromeos"
+target_os = "android"
+
+target_cpu = "arm"
+target_cpu = "x86"
+target_cpu = "x64"
+```
+
+See [GNCrossCompiles](cross_compiles.md) for more info.
+
 ## Configuring goma