Add .cdb.json

As documented here:
https://content.minetest.net/help/package_config/#cdbjson

.cdb.json

@@ -0,0 +1,12 @@
+{
+    "type": "GAME",
+    "title": "Alter",
+    "short_description": "A challenging puzzle game in a dystopian future.",
+    "dev_state": "BETA",
+    "tags": ["puzzle", "singleplayer", "oneofakind__original", "jam_game_2021"],
+    "license": "AGPL-3.0-or-later",
+    "media_license": "CC-BY-3.0",
+    "long_description": "Please follow the dialogue options in the lower left corner.",
+    "repo": "https://gitlab.com/yamanq/alter",
+    "issue_tracker": "https://gitlab.com/yamanq/alter/-/issues"
+}