git: openjdk/jdk11u-dev: 8251551: Use .md filename extension for README

[Christoph Langer]

Changeset: 4fbe635b
Author:    George Adams <georgeadams1995 at gmail.com>
Committer: Christoph Langer <clanger at openjdk.org>
Date:      2022-06-21 07:23:18 +0000
URL:       https://git.openjdk.org/jdk11u-dev/commit/4fbe635b21e3aba14dba760168a2f8965c7b29f5

8251551: Use .md filename extension for README

Reviewed-by: phh, clanger
Backport-of: 6ed221cb9ad2e81d92dda0ef32095dda5d52cb85

- README
+ README.md