chore: add selected screenshots to README.md

Signed-off-by: Robert Goniszewski <robertgoniszewski@outlook.com>

README.md

@@ -31,6 +31,15 @@ It's time to conjure up some organization! 📚✨
 - embrace the night with a dark mode 🌙
 - and stay productive using our official browser extension, _grimoire companion_ ([available here](https://github.com/goniszewski/grimoire-web-extension)) 🪄
 
+## Screenshots
+
+| Light Mode                                                                      | Dark Mode                                                                            |
+| ------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------ |
+| ![Only unread](screenshots/only-unread-white.png) Show only unread              | ![Adding new bookmark](screenshots/adding-new-bookmark-dark.png) Adding new bookmark |
+| ![User Profile view](screenshots/user-profile-view-white.png) User Profile view | ![Bookmark details](screenshots/bookmark-details-dark.png) Bookmark details          |
+
+More screenshots can be found in the [screenshots directory](screenshots).
+
 ## Installation
 
 <details>