Convert openneuro README to markdown

[mih]

The present on is in RST, which is "rendered" as plain text only.

[mih]

Not just openneuro. GIN doesn't like RST either: https://gin.g-node.org/studyforrest/visual-areas

[loj]

I'm currently starting on the OpenNeuro README, but all other dataset READMEs have been converted to markdown. #49

[loj]

I have the openneuro README converted. This could be updated on it's own now, or we could include it as part of the general dataset update that's planned. One argument for waiting is that the README includes file paths that may change as a result of incorporating the newly converted data.

[mih]

Thx, waiting makes sense!