Note testing branch as preferred for core contributions #357

[phillxnet]

As from git tag 4.1.0-0 we have begun extensive dependency update work in a new testing branch. Make this clear in developer docs and alter guidelines/instructions accordingly.

Fixes #357

Additionally includes

• minor typo fixes
• fix for malformed/mis-configured forum link

Checklist

• With the proposed changes no Sphinx errors or warnings are generated.
• I have added my name to the AUTHORS file, if required (descending alphabetical order).

[phillxnet]

@FroggyFlox If you have time, could you give these changes a quick glance over, just in case. I've kept the changes minimal here which I think is the way to go. From experience we mostly have already had contact with folks embarking on this endeavour so we really only need a concise guide here; hence avoiding a lot of background info with these additions.

In time we will need additional sections to address new build systems we hope to bring in but that is for another issue/pull-request set as and when.

[FroggyFlox]

That looks good to me, and a good addition!
I only have time for a quick look at it for the moment but it's indeed an important addition.

Thanks a lot for taking care of it!

[phillxnet]

@FroggyFlox Cheers and agreed. I'll merge and publish.

[phillxnet]

Patch product production published.