mac: Add link to mac_arm64.md from main macOS build instructions

Change-Id: If163eb94a69d8b3743658b84009027dbf80f1f86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514241
Auto-Submit: Reilly Grant <[email protected]>
Commit-Queue: Elly Fong-Jones <[email protected]>
Reviewed-by: Elly Fong-Jones <[email protected]>
Cr-Commit-Position: refs/heads/master@{#823592}
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index 4213fd64..b6609bb 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -104,6 +104,7 @@
   operating system and CPU.
 * For more info on GN, run `gn help` on the command line or read the
   [quick start guide](https://gn.googlesource.com/gn/+/master/docs/quick_start.md).
+* Building Chromium for arm Macs requires [additional setup](mac_arm64.md).
 
 
 ### Faster builds