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

Changes: https://git.openjdk.org/jdk11u/pull/47/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u&pr=47&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/pull/47.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u pull/47/head:pull/47

PR: https://git.openjdk.org/jdk11u/pull/47