8251551: Use .md filename extension for README

[gdams]

backports https://bugs.openjdk.org/browse/JDK-8251551 as it's a low risk change

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8251551: Use .md filename extension for README

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk11u pull/47/head:pull/47
$ git checkout pull/47

Update a local copy of the PR:
$ git checkout pull/47
$ git pull https://git.openjdk.org/jdk11u pull/47/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 47

View PR using the GUI difftool:
$ git pr show -t 47

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk11u/pull/47.diff

[bridgekeeper]

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[mlbridge]

Webrevs

• 01: Full - Incremental (61a3c4c7)
• 00: Full (ea6e99a7)

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[phohensee]

Unless this is a critical fix, it needs to be against jdk11u-dev rather than jdk11u.

[gdams]

Unless this is a critical fix, it needs to be against jdk11u-dev rather than jdk11u.

@phohensee apologies - I've moved the PR to openjdk/jdk11u-dev#1161