RFR: Add section on pre-built binaries to README [v2]

[Jorn Vernee]

> 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.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jextract/pull/84/files
  - new: https://git.openjdk.org/jextract/pull/84/files/4a8f808f..1ba7f75e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=84&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=84&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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