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

[Alex Ciminian]

On Fri, 12 Jun 2020 13:58:43 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> 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.

��  This solved my problem. I was using eclipse-photon as the target platform instead of `jmc-target-2020-03`, after
changing this the build succeeded.

I was able to complete all the steps successfully with a fresh install of Eclipse.

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