Add a worlds_here.txt file in the worlds folder and update .gitignore (#12720)

.gitignore

@@ -47,11 +47,9 @@ build/.cmake/
 /screenshots
 /sounds
 /mods/*
-!/mods/minetest/
-/mods/minetest/*
-!/mods/minetest/mods_here.txt
-/worlds
-/world/
+!/mods/mods_here.txt
+/worlds/*
+!/worlds/worlds_here.txt
 /clientmods/*
 !/clientmods/preview/
 /client/mod_storage/

worlds/worlds_here.txt

@@ -0,0 +1,4 @@
+World directories are placed into this location. Each world should contain at least following files:
+
+ * map_meta.txt
+ * world.mt