Thanks for bringing this up. This happens because you placed your Hologram code in the app directory, and elixirc_paths/1 defines where the Elixir compiler looks for source files. By default, it only includes lib, so adding app tells the compiler to also look there.
This step isn’t strictly necessary if you keep your code in the standard lib directory, but I’ve noticed many people (myself included! 
) prefer organizing Hologram pages/components in a separate app directory. Since this pattern is fairly common, I added it as an optional step in the Installation Guide.
