Additional explanation/image regarding bias nodes

[qualiaMachine]

I taught this workshop last week, and learners were very curious about the purpose of the bias nodes. I think it would be nice to add a short text snippet explaining what the bias nodes do -- maybe right after the image of a single neuron in the Introduction section. Something along the lines of:
"The bias nodes change the threshold of each neuron, i.e., they shift the activation function left or right."

An image showing the effect of bias would also be really helpful. Here's a link to an image I found that might work.

I'm happy to attempt this change via a pull request, but I wanted to check if this image is OK to use first. I think the image is by the author of this blog post, but I'm not positive.

[dafnevk]

I would suggest to add this as an (optional) call out box. My experience teaching is that we don't even get into the details of the activation function, but this probably depends very much on the crowd of learners and it could be useful optional material.