Copy the description from the script's `/help` output with some small
modifications for new viewers.

Include a demo screencast to show the script in action.

Copy the help description (since it's already written) to give users who
haven't installed the script an idea of what it does and how it works.

Add copyright and license information.

When I removed the `/vimput` command in
84ab68f96634cc29eb63a266a30b3147fec835e5, it broke help completion. You
were no longer able to get to our help from:

    /help vi<Tab>

This was because I had removed the command, and Irssi relies on commands
in order to set up completion for `/help`.

To hack completion into `/help`, use Irssi's subcommand feature to
provide a custom `/help vimput` subcommand to `/help`, and have it
display our help as before.

It doesn't really make sense to have a `/vimput` command since the
script modifies the input line. It's intended to be run on an existing
input line, not have you enter a command on the prompt.

Instead, change the command to a normal subroutine and instruct users in
the 'help' to create a binding to the Perl function in order to execute
the script.

Thanks to tmux-nicklist-portable for this idea:
https://github.com/irssi/scripts.irssi.org/blob/a0e86203562bac00ecc271842d6f4b238dae1c81/scripts/tmux-nicklist-portable.pl

Irssi help accessible from `/help vimput`. Describes the command and
lets users know that they should bind a shortcut in order to use the
plugin.

Turns out it's possible to make this a command instead of listening for
a shortcut on key press.

Now when you want to run Vimput, you can make your own binding instead
of me forcing C-x:

    /bind ^X /vimput