Mac GN doc.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3906
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=3906

Change-Id: I1ad13dd5b7df3d76f99f11e02f04169c5b37ce95
Reviewed-on: https://skia-review.googlesource.com/3906
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/site/user/quick/gn.md b/site/user/quick/gn.md
index abec361..c35b519 100644
--- a/site/user/quick/gn.md
+++ b/site/user/quick/gn.md
@@ -92,6 +92,11 @@
     adb push resources /data/local/tmp
     adb shell "cd /data/local/tmp; ./dm --src gm --config gpu"
 
+Mac
+---
+
+Mac users may want to pass `--ide=xcode` to `gn gen` to generate an Xcode project.
+
 Windows
 -------