Mention GN in the Linux build instructions documentation

There's no pointers to what GN is or how to set it up from this document
despite there being references to using it. This change adds the
GN Quick Start Guide to the Advanced Features section.

Review URL: https://codereview.chromium.org/1806213002

Cr-Commit-Position: refs/heads/master@{#381666}

docs/linux_build_instructions.md

@@ -132,6 +132,8 @@ page.
 
 ## Advanced Features
 
+*   Want to use the (in development) GN build workflow? See
+    [GN Quick Start](../tools/gn/docs/quick_start.md)
 *   Building frequently? See [LinuxFasterBuilds](linux_faster_builds.md).
 *   Cross-compiling for ARM? See [LinuxChromiumArm](linux_chromium_arm.md).
 *   Want to use Eclipse as your IDE? See