[foreign-jextract] RFR: Update the README with Panama specific information, and information about the repo

[Jorn Vernee]

Hi,

This patch updates the README file on the foreign-jextract branch, since that is features prominently on GitHub.

It also renames the file to README.md, so that GitHub picks up the right formatting.

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Commit messages:
 - Update the readme with Panama specific information, and information about the repo

Changes: https://git.openjdk.java.net/panama-foreign/pull/51/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/51/webrev.00
  Stats: 46 lines in 2 files changed: 34 ins; 12 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/51.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/51/head:pull/51

PR: https://git.openjdk.java.net/panama-foreign/pull/51