RFR: 6835: Adding developer guide for getting started with development

[Marcus Hirt]

On Fri, 12 Jun 2020 13:57:04 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> I have the active platform set to this:
>> 
>> ![Screenshot 2020-06-12 at 13 55
>> 18](https://user-images.githubusercontent.com/348973/84505203-39fa8980-acb5-11ea-8815-cfd4663d0805.png)
>> I can build `core` without problems so I guess that's fine?
>> 
>> I've also tried the 2020-03 configuration but that was yielding many more errors, so I switched back to this.
>
> Hmm, you should not have any problems with 2020-03. Photon is in queue for getting dropped eventually.
> 
> ![image](https://user-images.githubusercontent.com/16906077/84510354-51da0980-acc5-11ea-964f-865a22eb6dd3.png)

Try opening the platform file for 2020-03, let if find and resolve everything, and then click set as active platform.

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PR: https://git.openjdk.java.net/jmc/pull/82