Add a simple readme

README.txt

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+slay with friends
+=================
+
+multiplayer browser version of slay the spire, played with friends.
+
+based on the slay the spire board game rules from contention games.
+video game card art for visuals, board game mechanics for gameplay.
+
+1-4 players cooperate through map rooms, building decks and fighting
+enemies. if any player hits 0 HP, everyone loses.
+
+status: early development. building the core combat engine.
+
+setup
+-----
+
+requires bun (https://bun.sh)
+
+  bun install
+  bun run dev
+
+open http://localhost:3000
+
+development
+-----------
+
+  bun test          run tests
+  bun run check     lint + format + tests