If this relates to GitHub - crystal-lang/crystal-readline: Crystal bindings to GNU Readline Library then the readme could guide newcomers a little better.
GitHub - crystal-lang/crystal-sqlite3: SQLite3 bindings for Crystal suffers from the same issue, but maybe is more universally available.
Since you are dealing with readline you might be interested in a pure crystal implementation New shard REPLy : A Reader for your REPL