GEMINI.md: Add notes about 'shac'

Change-Id: Ibef547d5495141e1718c508b156ce3885b5e4110
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/306372
Reviewed-by: Oliver Newman <olivernewman@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>

GEMINI.md

diff --git a/GEMINI.md b/GEMINI.md
index bf4fe21..33c380f 100644
--- a/GEMINI.md
+++ b/GEMINI.md
@@ -92,3 +92,5 @@
 Testing
 - After making a change, run './presubmit.py' to check that everything still
   works
+- Also run 'shac fmt' to adjust any formatting
+- Run 'shac check' to check for lint issues