[jdk8u-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 and generally improves the readability/usability in GitHub.

Currently, I've just converted the README to markdown format and added a little syntax highlighting. I'm not sure if people would like me to go one step further and rip out the mercurial/nested repo references?

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Commit messages:
 - Backport 6ed221cb9ad2e81d92dda0ef32095dda5d52cb85

Changes: https://git.openjdk.org/jdk8u-dev/pull/75/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8251551
  Stats: 91 lines in 2 files changed: 51 ins; 40 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/75/head:pull/75

PR: https://git.openjdk.org/jdk8u-dev/pull/75