RFR: 8251551: Use .md filename extension for README

[Erik Helin]

Hi all,

please review this patch that adds the .md filename extension to the 
README file. The README already utilizes the Markdown [0] markup 
language even though it doesn't use the .md filename extension.

I also tweaked the Markdown syntax slightly, I primarily made use of 
Markdown for the links. Source code forges that support rendering README 
files (e.g. GitLab, GitHub, BitBucket, SourceHut, Gitea) will now 
display the README a bit better. For an example, see my personal fork 
[1] and compare it with how the current README is displayed [2].

Issue:
https://bugs.openjdk.java.net/browse/JDK-8251551

Webrev:
http://cr.openjdk.java.net/~ehelin/8251551/00/

Thanks,
Erik

[0]: https://en.wikipedia.org/wiki/Markdown
[1]: https://github.com/edvbld/jdk/tree/readme
[2]: https://git.openjdk.java.net/jdk