docs(readme): show the store screenshots (#24)

Adds the five phone screenshots from `fastlane/metadata/android/en-US/images/phoneScreenshots/` to the README header, as a centered row beneath the badges.

The README references the fastlane files directly rather than copies, so the README and the store listings stay in sync. Each image has descriptive alt text.

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/agendula/pulls/24

README.md

@@ -20,6 +20,14 @@ required.</p>
 <a href="https://ko-fi.com/jeanlucmakiola"><img src="https://storage.ko-fi.com/cdn/brandasset/v2/support_me_on_kofi_badge_beige.png" alt="Support me on Ko-fi" height="56"></a>
 </p>
 
+<p>
+<img src="fastlane/metadata/android/en-US/images/phoneScreenshots/01.png" alt="Home screen: today's progress, overdue and upcoming tasks, and your lists" width="18%">
+<img src="fastlane/metadata/android/en-US/images/phoneScreenshots/02.png" alt="Onboarding: sync with your own CalDAV server" width="18%">
+<img src="fastlane/metadata/android/en-US/images/phoneScreenshots/03.png" alt="Upcoming list with a task expanded into its subtasks" width="18%">
+<img src="fastlane/metadata/android/en-US/images/phoneScreenshots/04.png" alt="Task detail: a monthly recurring task with a reminder and priority" width="18%">
+<img src="fastlane/metadata/android/en-US/images/phoneScreenshots/05.png" alt="Reminder notification with Done and Snooze actions" width="18%">
+</p>
+
 </div>
 
 Agendula is the task-list sibling to [Calendula](https://codeberg.org/jlmakiola/calendula).