Updates install instructions on Debian/Ubuntu
Updates install instructions on Debian/Ubuntu

It looks like in (at least some) Debian distros, `xwd` is not available as the package `x11-apps` is not installed. As a consequence, after executing `ttygif`, a `xwd command not found` error will show up. Adding the package to the list of dependencies will solve the issue.
icholy/ttygif,icholy/ttygif