feat(library): ask whether a game is finished

README.md

@@ -33,6 +33,9 @@ To Be Played is split into three repositories:
   a link to the game's speedrun.com page when it has verified runs, a hint when Steam shows recent playtime for a
   game still in the backlog, and one to set the status to 100 % when
   every achievement is unlocked.
+- **Finished?**: Core marks a game as finished by itself when the ending achievement is unlocked and the status was
+  never set by hand. Otherwise the game page asks: "You unlocked the ending, finished?" or "You played longer than
+  most players need, finished?", with "Mark as finished" and "Not yet".
 - **Hide games**: a Steam game (e.g. a playtest or a tool) can be hidden on its detail page. It leaves the lists,
   the dashboard, the stats and the public profile; the library's "Hidden" filter shows it again for unhiding.
 - **Wishlist** with store price and discount.