RFR: Add section on pre-built binaries to README

Fri Oct 7 11:55:16 UTC 2022

I've added a short section on where to find the pre-built binaries. I think building jextract is less important than the information on usage if people can get binaries, so I've moved the section on building and testing to the bottom.

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Commit messages:
 - Spelling
 - Use markdown link for link to jextract binary
 - Add section on pre-built binaries

Changes: https://git.openjdk.org/jextract/pull/84/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=84&range=00
  Stats: 109 lines in 1 file changed: 58 ins; 47 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/84.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/84/head:pull/84

PR: https://git.openjdk.org/jextract/pull/84