Reformat readme for markdown and reword it

[metov]

The current readme doesn't look very nice on Github (see #475); this PR improves on it:

• Reformat with Markdown syntax as necessary
• Add .md extension (otherwise Github will render it as plain text)
  • Do we want to switch other files, like LICENSE, to .md as well for completeness?
• Reword content to remove some redundant phrasing and formatting

I realize the last point in particular is subjective, so please let me know if my edits are too drastic. I felt that since Markdown formatting opens some possibilities for simplifying the text, I might as well clean it up a little. I can revert those changes and turn it into a pure reformat if the reviewers prefer.

[metov]

Given the amount of time that has passed, and the fact that there are other competing PRs for the same issue, I will close this PR to avoid it going stale. Feel free to reopen if you decide to consider it.