Added SPA checklist

spa-checklist.md

+# Single Page App Checklist
+
+This is a checklist for browser-based Web App **without the backend** or with a 3rd-party backend.
+
+## Deployment
+ - My app is served from CDN with correct cache headers
+
+## Versioning
+ - My app has a cache-busting implemented 
+
+
+## Testing
+ - My app is passing Page Speed tests with high grades
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