Add more docs.

README.md

@@ -86,7 +86,11 @@ to have the following configured:
   service cloud.firestore {
     match /databases/{database}/documents {
       match /{document=**} {
+        // The gallery is globally readable.
         allow read: if true;
+
+        // We don't yet support submitting to the gallery, so
+        // deny all writes for now.
         allow write: if false;
       }
     }

lib/pages/gallery-page.tsx

@@ -33,6 +33,10 @@ export const GalleryPage: React.FC<{}> = () => {
   return (
     <Page title="Gallery!">
       <div className="sidebar">
+        <p>
+          This gallery is a work in progress! You can't yet submit anything to
+          it, and we have no thumbnails either. It will improve over time.
+        </p>
         <button onClick={ctx.refresh} disabled={ctx.isLoading}>
           {ctx.isLoading ? "Loading\u2026" : "Refresh"}
         </button>