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

[Alex Ciminian]

On Wed, 10 Jun 2020 20:08:38 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.

docs/devguide/README.md line 65:

> 64:
> 65: Next select File | Import… and select Maven / Existing Maven Project again, but this time from the root.
> 66:

I've been able to successfully follow the steps up to this point, but here I got an error:

<img width="410" alt="Screenshot 2020-06-10 at 23 04 34"
src="https://user-images.githubusercontent.com/348973/84486996-0e1bdb80-ac96-11ea-8ae0-5ace8308fd7d.png">

After importing the Maven projects from the jmc root directory I see compilation errors because of a missing jface
`ResourceLocator` path:

<img width="1171" alt="Screenshot 2020-06-11 at 00 31 22"
src="https://user-images.githubusercontent.com/348973/84487139-39062f80-ac96-11ea-823b-f15f4f4b00a2.png">

The jar is there but it looks like an old version: `org.eclipse.jface_3.14.0.v20180423-0714.jar`. I've tried manually
upgrading this jar and I've tried upgrading the plugin development environment but it didn't work - I don't know much
about eclipse, I need to dig into this a bit more.

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