RFR: 8251552: Add minimal CONTRIBUTING.md file

[Erik Helin]

On 8/14/20 5:19 PM, Iris Clark wrote:
> Hi, Erik.
> 
>> - full: https://cr.openjdk.java.net/~ehelin/8251552/01/
>> - incremental: https://cr.openjdk.java.net/~ehelin/8251552/00-01/
> 
> This revised version referencing only ojn:contribute looks good.

Thanks Iris for reviewing!
Erik

> Thank you!
> Iris