Add a how-to guide for writing a SciJava Op #183

I want to write a SciJava Op but I don't know where to start. The Writing Your Own Op Package section contains some useful SciJava Ops info, but I'm still confused. I'd like to see a step-by-step guide (i.e. it should be numbered) on how to write a SciJava Op.

I recently went through the napari plugin tutorial and got my first basic plugin going pretty quickly. What I liked about this experience was that I could quickly browse through the various steps needed to get something going. This gave me an idea of the complexity of writing a napari plugin. I think this could be a great selling point for SciJava Ops as we want to really push that its easier now with SciJava Ops.