[foreign-jextract] [Rev 01] 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.

Jorn Vernee has updated the pull request incrementally with two additional commits since the last revision:

 - Fixing some typos
 - Review comments

-------------

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/51/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/51/files/c007f72e..36a7a5fc

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/51/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/51/webrev.00-01

  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 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