docs: add contributing guidelines and update README for environment variables

- Created a new `CONTRIBUTING.md` file outlining development setup, testing, and commit message conventions.
- Updated `README.md` to clarify environment variable usage, including new variables for HTTP port, CORS origins, and duty display timezone.
- Enhanced documentation for bot commands and API endpoints to improve user understanding and onboarding.
- Added example configurations in `.env.example` for better clarity on optional settings.

.env.example

@@ -22,3 +22,9 @@ ADMIN_USERNAMES=admin1,admin2
 
 # Default UI language when user language is unknown: en or ru (default: en).
 # DEFAULT_LANGUAGE=en
+
+# Reject Telegram initData older than this (seconds). 0 = do not check (default).
+# INIT_DATA_MAX_AGE_SECONDS=0
+
+# Comma-separated CORS origins; leave unset for *.
+# CORS_ORIGINS=