[code-reflection] Integrated: updates README.md to include fix for backend cmake

[Gary Frost]

README fixup.

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Commit messages:
 - updates README.md to include fix for backend cmake

Changes: https://git.openjdk.org/babylon/pull/129/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=129&range=00
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/129.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/129/head:pull/129

PR: https://git.openjdk.org/babylon/pull/129