RFR: 8257776: [valhalla:jep390] Add disclaimer about future changes to value-based classes

[Dan Smith]

Addressing CSR feedback by mentioning the possibility of future behavioral changes to value-based classes.

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

Commit messages:
 - 8257776: [valhalla:jep390] Add disclaimer about future changes to value-based classes

Changes: https://git.openjdk.java.net/valhalla/pull/294/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=294&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257776
  Stats: 81 lines in 45 files changed: 32 ins; 0 del; 49 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/294.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/294/head:pull/294

PR: https://git.openjdk.java.net/valhalla/pull/294