Add a reference to how it works for simple PRs

This can be helpful to understand what's going on under the hood.

I was trying to explain how spr works to a coworker and it got kind of confusing. I went looking for an explanation, but didn't see one. So, I decided to throw this together in case it would be helpful for anyone else.

This is based on my understanding of what happens from both using spr and from reading the code. This might not be the most accurate thing, so any improvements are welcome.

This uses the Gitgraph diagram of Mermaid.js to show what happens with git. This should render on GitHub, since they added support for Mermaid.js not too long ago.

Since the spr website uses mdBook, it should be possible to use the mdbook-mermaid preprocessor to render these diagrams as well. However, I haven't tested this yet personally.

If there's a want for this kind of documentation, explaining how the stacked workflow works will probably be even more useful. It's a bit more complex, so understanding that can be very helpful to know what's happening. I didn't add that right away, since it's a non-trivial amount of work so I wanted to make sure this kind of documentation is even wanted first.

Test Plan: This is documentation, so aside from it making sense and being wanted the test is whether or not it renders properly.

spacedentist / spr

Add a reference to how it works for simple PRs #125