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

[Alex Ciminian]

On Fri, 12 Jun 2020 12:31:08 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> This PR adds a short developer guide, to help newcomers getting started setting up the development environment.
>
> Hmm. Interesting. And you did set the active platform properly?

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.

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