This pull request updates README.md:

• Replaces clear with a POSIX-compliant function,
• Fixes the ShellCheck warning SC2235 in vterm_printf() ("Use { ..; } instead of (..) to avoid subshell overhead")

  You appear to be using (..) to group test commands. This creates a subshell, making it unnecessarily slow. Avoid this by using { ..; } to group.
  Be careful to note that unlike (..), this requires both a space after the { and a semicolon before the }.
  For example, (cmd), (cmd;) and ( cmd ) are all valid, but {cmd}, {cmd;} and { cmd } are all syntax errors because they lack either or both of the spaces and semicolon. The correct form is { cmd; }