Increase limit to 500 entries and warn about limit.

fedireads/templates/import.html

@@ -8,6 +8,8 @@
         {{ import_form.as_p }}
         <button type="submit">Import</button>
     </form>
+    <p>
+    Imports are limited in size, and only the first {{ limit }} items will be imported.
 
     <h2>Recent Imports</h2>
     <ul>