docs: add data structure section to README with column mappings for Google Sheet

Author: githyperplexed

README.md

@@ -79,6 +79,19 @@ Run the generation script:
 bun run gen
 ```
 
+### Data Structure
+
+The application expects specific columns in the source Google Sheet. These are mapped to short keys to minimize the JSON file size.
+
+| Sheet Column Name | Key | Description |
+|-------------------|-----|-------------|
+| Name | `n` | Store Name |
+| URL | `u` | Store URL (https:// stripped) |
+| Shoppers 30d | `s30` | Shoppers count (30 days) |
+| Avg Savings 30d | `a30` | Average savings (30 days) |
+| Affiliate URL | `p` | Has affiliate link (1 = Yes, 0 = No) |
+| Number of Visible Coupons | `c` | Coupon count |
+
 ### Running the App
 
 Start the development server: