Enhancing tutorial with 4th step for smooth transition to new amp value

[lossius]

Thanks for a great initiative. I'm looking forward to using this for my own projects, and I also hope that I'll find the time to contribute from time to time as well. Having developed the Ambisonic Toolkit for Reaper plugins, I am now in need of a series of multichannel effects, and it seems a really good idea to build them on top of your library.

Below are some additions to the first tutorial, illustrating how to make gain values ramp smoothly in order to avoid clicks. In particular for newcomers to programming, DSP processing and JSFX this might be useful info, and for me personally it's a first rehearsal into doing forks and pull requests, I haven't done that before.