Fix broken link and add cross-compiling to quick start

tools/gn/docs/quick_start.md

 # GN Quick Start guide
 
 [TOC]
 
 ## Running GN
 
 You just run `gn` from the command line. There is a script in
 depot\_tools (which is presumably on your path) with this name. The
 script will find the binary in the source tree containing the current
 directory and run it.
@@ skipping 32 matching lines @@
 
 ```
 gn args --list out/my_build
 ```
 
 on the command line. See "Taking build arguments" below for information
 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"

[brettw, 2015/06/19 17:47:06]

Can you put spaces on each side of the = in these lines?

[dmazzoni, 2015/07/16 19:30:02]

Done.

+target_os="android"
+
+target_cpu="arm"
+target_cpu="x86"
+target_cpu="x64"
+```
+
+See [GNCrossCompiles](cross_compiles.md) for more info.
+
 ## Configuring goma
 
 
 Run `gn args out/Default` (substituting your build directory as needed).
 Add:
 
 ```
 use_goma = true
 goma_dir = "~/foo/bar/goma" 
 ```
@@ skipping 257 matching lines @@
 line flag. This will output a summary of timings for various things.
 
 You can also make a trace of how the build files were executed:
 
 ```
 gn --tracelog=mylog.trace
 ```
 
 and you can load the resulting file in Chrome's `about:tracing` page to
 look at everything.