I've grabbed Nathan's patch, and pushed it to GitHub simply to preview the rendered Markdown there. This isn't intended to be reviewed, this is just for anyone else that's interested in easily seeing the HTML version of the Markdown file compared with the earlier one.
I agree that starting with the direct conversion is reasonable. Markdown "modernizes" the file using a popular plain text file format that's renderable.
However, I also think it would be cool to get some input on what the most useful 2-3 content items are for new developers and make any additions possible there. In writing this, I had an idea to ask about whether this topic could be covered as an upcoming PostgreSQL community blog post series. In theory, we could gather a variety of perspectives that way. That could make it less subjective if we see several people independently suggesting a particular wiki page for example, for inclusion in the README. I'll pursue that outside the mailing list and report back!