add downloads badge

[ranocha]

This is just a test/suggestion. We can discuss whether we like it or not.

CC @andrewwinters5000 @jlchan @gregorgassner @sloede

The badge looks as follows:

[jlchan]

Approved, but maybe discuss at meeting before merging?

[sloede]

Thanks for suggesting this, @ranocha! However, I wonder what the purpose of this badge is, and thus ultimately, what the merit for users or developers of Trixi is:

To me, it seems like the purpose is akin to the GitHub stars, which allows visitors to indicate that they like a software package/repo and thereby earning developers some "bragging rights". IMHO, the statistics (number of packages installed based on numbers obtained from the Julia package servers) only make sense IMHO if they capture the actual usage of a package. Here I see a disadvantage for Trixi, since I think most of our users are actually clone Trixi and use it as devs, while fewer users just consider it a "user" package that is just installed and be done with it. For reference, we have ~800 package installs and about twice the number of clones (~1,500).

While the last assessment is also a statement in its own right (maybe we are not yet where we want to be in terms of usability and extensibility?), I then wonder what the number of downloads tells potential users beyond the raw number itself, since it does not seem to be a good proxy for "usage".

Having said that, I am not generally opposed to adding it to the README.