RFR: 8251551: Use .md filename extension for README

[Erik Helin]

On 8/14/20 11:43 AM, Magnus Ihse Bursie wrote:
> When I wrote the original README, the only use-case I had in mind was 
> for someone to have checked out the code locally, not someone seeing it 
> as a project description on Github. So I'd suggest a change more along 
> these lines:
> ---
> For build instructions, please see the [online 
> documentation](https://openjdk.java.net/groups/build/doc/building.html), 
> or either of these files:
> 
>   - [doc/building.html](doc/building.html) (html version)
>   - [doc/building.md](doc/building.md) (markdown version)
> ---

Thanks for reviewing Magnus! I agree with your suggestion, please see 
updated webrevs below:

- full: https://cr.openjdk.java.net/~ehelin/8251551/01/
- incremental: https://cr.openjdk.java.net/~ehelin/8251551/00-01/

You can also see a rendered version of the README.md file in my personal 
fork [0].

Thanks,
Erik

[0]: https://github.com/edvbld/jdk/blob/readme/README.md