Update README.md

tests/caveview/README.md

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 ### How to run it?
 
 There's no GUI. Find a directory with Minecraft Anvil files (.mca).
-Copy a Minecraft "terrain.png" into that directory. Run from that
-directory.
+Copy a Minecraft "terrain.png" into that directory (do a google
+image search). Run from that directory.
 
 ### How accurate is this as a Minecraft viewer?