Add post image credit

[pranavmalvawala]

Hi @scttcper, Love your site, awesome job.

I actually need your help in adding an image credit for blog post images using the same markdown package we are using - rehype-react. I tried to use it but I couldn't find my way around it, so I had to use react-markdown but I don't wanna use a whole package just to add this link. You can find my implementation here https://pranavmalvawala.com/github-link, End result would look like this

I propose that we add a new field in markdown file like

imageCredit: 'Photo by [Callum Shaw](https://unsplash.com/@callumshaw)'
and then we query it in post.tsx template and just add it below that image.

Let me know what you think man.