[JOSS 6532] Expand and clarify contribution guidlines

[dfm]

One of the JOSS review requirements is that submissions have clear contribution guidelines, but I couldn't find them documented for JAXBind. This should probably include developer docs and info about issue/PR expectations, for example.

ref: openjournals/joss-reviews#6532

[Edenhofer]

Thanks for your comment! Is #20 sufficient or shall the contributions guide be more detailed?

[dfm]

I'd say that that does satisfy the letter of the JOSS requirements, but perhaps doesn't present the most welcoming face to new developers. I leave it up to you to decide if this is sufficient for your purposes!

[Edenhofer]

Thanks! You're right. The section reads really grumpy. I probably woke up on the wrong side of the bed yesterday 😬

Hope it reads much friendlier now :)