Add custom components auto docs to README.md as well

[abidlabs]

When we publish a custom component, we should write the auto-generated documentation to the README.md file as well, not just to the space.py file. As the README.md becomes the default documentation if they publish their component to GitHub, which may often be the case.

For example, I just created a new custom component: HighlightedCode.

Spaces shows the full documentation:

But the README.md is quite sparse:

[abidlabs]

That's because I forgot to push my changes 😵‍💫

Amazing stuff @pngwn @freddyaboulton