[jdk11u-dev] RFR: 8251551: Use .md filename extension for README

[George Adams]

backports https://bugs.openjdk.org/browse/JDK-8251551 as it's a low risk change

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Commit messages:
 - Backport 6ed221cb9ad2e81d92dda0ef32095dda5d52cb85

Changes: https://git.openjdk.org/jdk11u-dev/pull/1161/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1161&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8251551
  Stats: 23 lines in 2 files changed: 11 ins; 12 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1161.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1161/head:pull/1161

PR: https://git.openjdk.org/jdk11u-dev/pull/1161