Add calendar_node readme

calendar_node/README.md

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+# Calendar Node
+
+This mod adds a placable and craftable calendar.
+(See the crafting guide to see how to craft it).
+
+Provided you have the `calendar` mod, you can use the calendar to view
+the current date in the HUD.
+
+See the `calendar` mod to learn more about the calendar system.
+
+If the `calendar` mod is not found, the placable calendar is basically just
+a decorative block.