migrate README to markdown

[pmeier]

It always bugged me that we had our README in rst rather than in markdown. This usually is only done in case we need to pull this into our sphinx documentation. But this is not the case here. Thus, I've aligned the README with all of our other files and migrated it to markdown.

This PR is a 1-to-1 port except for two changes:

1. I've marked all headers below the first one as H2 rather than H1, which is usually reserved for the title.
2. I've added a collapsible section to our compatibility table, since it has grown pretty large.

Rendered README.md

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/7565

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[NicolasHug]

I think the reason this is a rst file comes from the good old days when PyPI didn't support markdown (and IIRC GitHub didn't support rst for README. Good times).

Anyway, IMHO this is a very marginal improvement but I don't have a strong opinion so I'll approve to unblock. FYI, the collapsed table is formatted as "raw" - not sure if this is intended.

[pmeier]

FYI, the collapsed table is formatted as "raw" - not sure if this is intended.

Nope, not intended.

[github-actions]

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