[jdk8u-dev] RFR: 8251551: Use .md filename extension for README

[Zdenek Zambersky]

This backport adds `.md` extension to README. It also changes formulations a bit and adds some hyperlinks. Backport is not clean because there were small intermediate changes to README by (changesets not applicable to JDK8):
- [JDK-8187444](https://bugs.openjdk.org/browse/JDK-8187444)
- [JDK-8205956](https://bugs.openjdk.org/browse/JDK-8205956)

(Git however considers this rewrite anyway.)

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Commit messages:
 - Backport 4fbe635b21e3aba14dba760168a2f8965c7b29f5

Changes: https://git.openjdk.org/jdk8u-dev/pull/394/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=394&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8251551
  Stats: 21 lines in 2 files changed: 11 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/394.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/394/head:pull/394

PR: https://git.openjdk.org/jdk8u-dev/pull/394