[foreign-jextract] RFR: Update README

[Maurizio Cimadamore]

I've tweaked the README file of the foreign-jextract branch to reflect the changes in the branch organization of the repository.

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Commit messages:
 - Update README

Changes: https://git.openjdk.java.net/panama-foreign/pull/405/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=405&range=00
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/405.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/405/head:pull/405

PR: https://git.openjdk.java.net/panama-foreign/pull/405