[foreign-jextract] RFR: 8241349: initial version of panama_jextract.md

[Athijegannathan Sundararajan]

adding initial version panama_jextract.md

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Commit messages:
 - 8241349: initial version of panama_jextract.md

Changes: https://git.openjdk.java.net/panama-foreign/pull/61/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/61/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241349
  Stats: 503 lines in 2 files changed: 503 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/61.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/61/head:pull/61

PR: https://git.openjdk.java.net/panama-foreign/pull/61