Goal: add one concise Usage section to README for new contributors.\n\nScope:\n1. Keep existing Quick start section unchanged.\n2. Add a new ## Usage section after Quick start.\n3. Include one short fenced code block showing how to call Println from main/main.neva.\n4. Keep README concise (no more than ~15 lines total).\n\nAcceptance criteria:\n- README contains ## Usage with exactly one code block.\n- Existing Quick start steps remain present and readable.\n- No changes outside README are required.