RFR: 8251551: Use .md filename extension for README

[mark.reinhold at oracle.com]

2020/8/14 4:57:18 -0700, erik.helin at oracle.com:
> 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].

There’s an extra comma in there.

s/For build instructions, please/For build instructions please see/

Otherwise, looks good.

- Mark