[amber-docs] RFR: Apply markdown formatting on GitHub

[Tesla I. Zhang‮]

Renamed README to README.md so the file will be displayed with markdown rendering on GitHub.

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Commit messages:
 - Apply markdown formatting on GitHub

Changes: https://git.openjdk.java.net/amber-docs/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/?repo=amber-docs&pr=17&range=00
  Stats: 0 lines in 1 file changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/amber-docs/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/amber-docs pull/17/head:pull/17

PR: https://git.openjdk.java.net/amber-docs/pull/17