JOSS: address reviews

[elboyran]

Address the JOSS reviews

[gcroci2]

I opened PR #355

[elboyran]

@gcroci2, @egpbos, @cwmeijer , @loostrum, @geek-yang would any/some of you have time to address also the unchecked check boxes by each of the 2 reviewers (despite the very uninformative reviews...)? And also the automated editorialbot suggestions?

[egpbos]

So

1. an installation instruction should be added (pip install dianna, basically? the reviewer says "for newbies", so maybe they also want to see that you first must install python, pip and only then install dianna?) and
2. a "functionality" explanation should be added, so I guess a basic usage example?
3. Depending on whether sara-02 can run the tests now, something may need to be added there as well?

[cwmeijer]

Outdated and duplicate of #367 .